kunit: make kunit_tool accept optional path to .kunitconfig fragment

Currently running tests via KUnit tool means tweaking a .kunitconfig
file, which you'd keep around locally and never commit.
This changes makes it so users can pass in a path to a kunitconfig.

One of the imagined use cases is having kunitconfig fragments in-tree
to formalize interesting sets of tests for features/subsystems, e.g.
  $ ./tools/testing/kunit/kunit.py run --kunticonfig=fs/ext4/kunitconfig

For now, this hypothetical fs/ext4/kunitconfig would contain
  CONFIG_KUNIT=y
  CONFIG_EXT4_FS=y
  CONFIG_EXT4_KUNIT_TESTS=y

At the moment, it's not hard to manually whip up this file, but as more
and more tests get added, this will get tedious.

It also opens the door to documenting how to run all the tests relevant
to a specific subsystem or feature as a simple one-liner.

This can be seen as an analogue to tools/testing/selftests/*/config
But in the case of KUnit, the tests live in the same directory as the
code-under-test, so it feels more natural to allow the kunitconfig
fragments to live anywhere. (Though, people could create a separate
directory if wanted; this patch imposes no restrictions on the path).

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