doc: unit test runner documentation fixup

Running `test_bitcoin --help` prints the list of arguments that may be passed,
not the list of tests.

Author: jonatack

src/test/README.md

@@ -97,7 +97,7 @@ If you run an entire test suite, such as `--run_test=getarg_tests`, or all the t
 (by not specifying `--run_test`), a separate directory
 will be created for each individual test.
 
-Run `test_bitcoin --help` for the full list of tests.
+Run `test_bitcoin --help` for the full list of arguments that can be passed.
 
 ### Adding test cases