Make the change backwards-compatible

This supports both the old version (single value) and new version (hash-based).

I've added back the tests from the old version to check that the code stills works
when we provide a single value.

I've also adapted the README a bit to explain both versions.

README.md

@@ -121,15 +121,15 @@ class MyWidget
       e.try { UserService.slug_from_login login }  # returns String instance or ArgumentError
 
       compare_error_message_and_class = -> (control, candidate) do
-        control.class == candidate.class && 
+        control.class == candidate.class &&
         control.message == candidate.message
       end
 
       compare_argument_errors = -> (control, candidate) do
         control.class == ArgumentError &&
         candidate.class == ArgumentError &&
         control.message.start_with?("Input has invalid characters") &&
-        candidate.message.start_with?("Invalid characters in input") 
+        candidate.message.start_with?("Invalid characters in input")
       end
 
       e.compare_errors do |control, candidate|
@@ -550,7 +550,25 @@ end
 
 #### Providing fake timing data
 
-If you're writing tests that depend on specific timing values, you can provide canned durations using the `fabricate_durations_for_testing_purposes` method, and Scientist will report these in `Scientist::Observation#duration` and `Scientist::Observation#cpu_time` instead of the actual execution times.
+If you're writing tests that depend on specific timing values, you can provide canned durations using the `fabricate_durations_for_testing_purposes` method. This can be done using either the old version (single value) or the new version (hash-based) to include both duration and cpu_time.
+
+##### Old version (Single Value)
+
+In the old version, you can provide a single value for the duration:
+
+```ruby
+science "absolutely-nothing-suspicious-happening-here" do |e|
+  e.use { ... } # "control"
+  e.try { ... } # "candidate"
+  e.fabricate_durations_for_testing_purposes( "control" => 1.0, "candidate" => 0.5 )
+end
+```
+
+`fabricate_durations_for_testing_purposes` takes a Hash of duration values, keyed by behavior names.  (By default, Scientist uses `"control"` and `"candidate"`, but if you override these as shown in [Trying more than one thing](#trying-more-than-one-thing) or [No control, just candidates](#no-control-just-candidates), use matching names here.)  If a name is not provided, the actual execution time will be reported instead.
+
+##### New version (Hash-based)
+
+Scientist will report these in `Scientist::Observation#duration` and `Scientist::Observation#cpu_time` instead of the actual execution times.
 
 ```ruby
 science "absolutely-nothing-suspicious-happening-here" do |e|

lib/scientist/observation.rb

@@ -35,9 +35,12 @@ def initialize(name, experiment, fabricated_duration: nil, &block)
       @exception = e
     end
 
-    if fabricated_duration
+    if fabricated_duration.is_a?(Hash)
       @duration = fabricated_duration["duration"]
       @cpu_time = fabricated_duration["cpu_time"]
+    elsif fabricated_duration
+      @duration = fabricated_duration
+      @cpu_time = 0.0 # setting a default value
     else
       end_wall_time, end_cpu_time = capture_times
       @duration = end_wall_time - start_wall_time

test/scientist/experiment_test.rb

@@ -671,7 +671,20 @@ def @ex.enabled?
   end
 
   describe "testing hooks for extending code" do
-    it "allows a user to provide fabricated durations for testing purposes" do
+    it "allows a user to provide fabricated durations for testing purposes (old version)" do
+      @ex.use { true }
+      @ex.try { true }
+      @ex.fabricate_durations_for_testing_purposes( "control" => 0.5, "candidate" => 1.0 )
+
+      @ex.run
+
+      cont = @ex.published_result.control
+      cand = @ex.published_result.candidates.first
+      assert_in_delta 0.5, cont.duration, 0.01
+      assert_in_delta 1.0, cand.duration, 0.01
+    end
+
+    it "allows a user to provide fabricated durations for testing purposes (new version)" do
       @ex.use { true }
       @ex.try { true }
       @ex.fabricate_durations_for_testing_purposes({
@@ -692,7 +705,20 @@ def @ex.enabled?
       assert_equal 0.9, cand.cpu_time
     end
 
-    it "returns actual durations if fabricated ones are omitted for some blocks" do
+    it "returns actual durations if fabricated ones are omitted for some blocks (old version)" do
+      @ex.use { true }
+      @ex.try { sleep 0.1; true }
+      @ex.fabricate_durations_for_testing_purposes( "control" => 0.5 )
+
+      @ex.run
+
+      cont = @ex.published_result.control
+      cand = @ex.published_result.candidates.first
+      assert_in_delta 0.5, cont.duration, 0.01
+      assert_in_delta 0.1, cand.duration, 0.01
+    end
+
+    it "returns actual durations if fabricated ones are omitted for some blocks (new version)" do
       @ex.use { true }
       @ex.try do
         start_time = Time.now