kunit: tool: make --kunitconfig accept dirs, add lib/kunit fragment

TL;DR
$ ./tools/testing/kunit/kunit.py run --kunitconfig=lib/kunit

Per suggestion from Ted [1], we can reduce the amount of typing by
assuming a convention that these files are named '.kunitconfig'.

In the case of [1], we now have
$ ./tools/testing/kunit/kunit.py run --kunitconfig=fs/ext4

Also add in such a fragment for kunit itself so we can give that as an
example more close to home (and thus less likely to be accidentally
broken).

[1] https://lore.kernel.org/linux-ext4/YCNF4yP1dB97zzwD@mit.edu/

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

lib/kunit/.kunitconfig

@@ -0,0 +1,3 @@
+CONFIG_KUNIT=y
+CONFIG_KUNIT_TEST=y
+CONFIG_KUNIT_EXAMPLE_TEST=y