kunit: add run_checks.py script to validate kunit changes

This formalizes the checks KUnit maintainers have been running (or in
other cases: forgetting to run).

This script also runs them all in parallel to minimize friction (pytype
can be fairly slow, but not slower than running kunit.py).

Example output:
$ ./tools/testing/kunit/run_checks.py
Waiting on 4 checks (kunit_tool_test.py, kunit smoke test, pytype, mypy)...
kunit_tool_test.py: PASSED
mypy: PASSED
pytype: PASSED
kunit smoke test: PASSED

On failure or timeout (5 minutes), it'll dump out the stdout/stderr.
E.g. adding in a type-checking error:
  mypy: FAILED
  > kunit.py:54: error: Name 'nonexistent_function' is not defined
  > Found 1 error in 1 file (checked 8 source files)

mypy and pytype are two Python type-checkers and must be installed.
This file treats them as optional and will mark them as SKIPPED if not
installed.

This tool also runs `kunit.py run --kunitconfig=lib/kunit` to run
KUnit's own KUnit tests and to verify KUnit kernel code and kunit.py
play nicely together.

It uses --build_dir=kunit_run_checks so as not to clobber the default
build_dir, which helps make it faster by reducing the need to rebuild,
esp. if you're been passing in --arch instead of using UML.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>