The new --run_isolated flag makes the tool boot the kernel once per
suite or test, preventing leftover state from one suite to impact the
other. This can be useful as a starting point to debugging test
hermeticity issues.

Note: it takes a lot longer, so people should not use it normally.

Consider the following very simplified example:

  bool disable_something_for_test = false;
  void function_being_tested() {
    ...
    if (disable_something_for_test) return;
    ...
  }

  static void test_before(struct kunit *test)
  {
    disable_something_for_test = true;
    function_being_tested();
    /* oops, we forgot to reset it back to false */
  }

  static void test_after(struct kunit *test)
  {
    /* oops, now "fixing" test_before can cause test_after to fail! */
    function_being_tested();
  }

Presented like this, the issues are obvious, but it gets a lot more
complicated to track down as the amount of test setup and helper
functions increases.

Another use case is memory corruption. It might not be surfaced as a
failure/crash in the test case or suite that caused it. I've noticed in
kunit's own unit tests, the 3rd suite after might be the one to finally
crash after an out-of-bounds write, for example.

Example usage:

Per suite:
$ ./tools/testing/kunit/kunit.py run --kunitconfig=lib/kunit --run_isolated=suite
...
Starting KUnit Kernel (1/7)...
============================================================
======== [PASSED] kunit_executor_test ========
....
Testing complete. 5 tests run. 0 failed. 0 crashed. 0 skipped.
Starting KUnit Kernel (2/7)...
============================================================
======== [PASSED] kunit-try-catch-test ========
...

Per test:
$ ./tools/testing/kunit/kunit.py run --kunitconfig=lib/kunit --run_isolated=test
Starting KUnit Kernel (1/23)...
============================================================
======== [PASSED] kunit_executor_test ========
[PASSED] parse_filter_test
============================================================
Testing complete. 1 tests run. 0 failed. 0 crashed. 0 skipped.
Starting KUnit Kernel (2/23)...
============================================================
======== [PASSED] kunit_executor_test ========
[PASSED] filter_subsuite_test
...

It works with filters as well:
$ ./tools/testing/kunit/kunit.py run --kunitconfig=lib/kunit --run_isolated=suite example
...
Starting KUnit Kernel (1/1)...
============================================================
======== [PASSED] example ========
...

It also handles test filters, '*.*skip*' runs these 3 tests:
  kunit_status.kunit_status_mark_skipped_test
  example.example_skip_test
  example.example_mark_skipped_test

Fixed up merge conflict between:
  d8c23ead ("kunit: tool: better handling of quasi-bool args (--json, --raw_output)") and
  6710951ee039 ("kunit: tool: support running each suite/test separately")
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
    Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: NDaniel Latypov <dlatypov@google.com>
Reviewed-by: NDavid Gow <davidgow@google.com>
Reviewed-by: NBrendan Higgins <brendanhiggins@google.com>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>