doc: unit test runner documentation improvements

including hoisting the `test_bitcoin --help` sentence to the beginning of the
section, where it is probably the most useful.

Author: jonatack

src/test/README.md

@@ -35,15 +35,26 @@ the `src/qt/test/test_main.cpp` file.
 
 ### Running individual tests
 
-`test_bitcoin` accepts the command line arguments from the boost framework.
-For example, to run just the `getarg_tests` suite of tests:
+The `test_bitcoin` runner accepts command line arguments from the Boost
+framework. To see the list of arguments that may be passed, run:
+
+```
+test_bitcoin --help
+```
+
+For example, to run only the tests in the `getarg_tests` file, with full logging:
 
 ```bash
 build/src/test/test_bitcoin --log_level=all --run_test=getarg_tests
 ```
 
-`log_level` controls the verbosity of the test framework, which logs when a
-test case is entered, for example.
+or
+
+```bash
+build/src/test/test_bitcoin -l all -t getarg_tests
+```
+
+The `--log_level=` (or `-l`) argument controls the verbosity of the test output.
 
 `test_bitcoin` also accepts some of the command line arguments accepted by
 `bitcoind`. Use `--` to separate these sets of arguments:
@@ -62,7 +73,7 @@ standard terminal output.
 build/src/test/test_bitcoin --run_test=getarg_tests/doubledash
 ```
 
-`test_bitcoin` creates a temporary working (data) directory with a randomly
+Running `test_bitcoin` creates a temporary working (data) directory with a randomly
 generated pathname within `test_common bitcoin/`, which in turn is within
 the system's temporary directory (see
 [`temp_directory_path`](https://en.cppreference.com/w/cpp/filesystem/temp_directory_path)).
@@ -97,8 +108,6 @@ 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 arguments that can be passed.
-
 ### Adding test cases
 
 To add a new unit test file to our test suite, you need