pw_unit_test: Add code coverage awareness

This changes `test_runner.py` to be aware of the intention to run a code
coverage test (any test that is executed with the automatic runner when
`pw_toolchain_SANITIZERS` contains `"coverage"`). This is accomplished
by adding an extra `argparse` arg called `--run-for-coverage`.

When that flag is set, the test runner will set the `LLVM_PROFILE_FILE`
environment variable to generate a `profraw` file next to the test
binary.

The `pw_test` template is also updated to check for `"coverage"` in
`pw_toolchain_SANITIZERS`, and sets `--run-for-coverage` and includes
the `profraw` file in outputs when in coverage mode.

The `profraw` file path for the test is then propagated through
`metadata.profraws` similar to `tests`. This allows a `generated_file`
to be used downstream to collect all `profraw` paths.

Further work is required to use these `profraw` files to actually
generate coverage reports with `llvm-cov` or some other tool, but this
is a first step.

Tested: `pw presubmit`.
Tested: Downstream project continues to build in coverage mode and succesfully outputs `profraw` files.
Change-Id: I098ecceae5cba27e8f7e5a129d71f358f301340a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/116317
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Greg Pataky <gregpataky@google.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>