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/[e-mail]/

Signed-off-by: Daniel Latypov <[e-mail]>
Reviewed-by: David Gow <[e-mail]>
Reviewed-by: Brendan Higgins <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

lib/kunit/.kunitconfig

···
+CONFIG_KUNIT=y
+CONFIG_KUNIT_TEST=y
+CONFIG_KUNIT_EXAMPLE_TEST=y

tools/testing/kunit/kunit.py

···
 			    help='Run all KUnit tests through allyesconfig',
 			    action='store_true')
 	parser.add_argument('--kunitconfig',
-			     help='Path to Kconfig fragment that enables KUnit tests',
+			     help='Path to Kconfig fragment that enables KUnit tests.'
+			     ' If given a directory, (e.g. lib/kunit), "/.kunitconfig" '
+			     'will get  automatically appended.',
 			     metavar='kunitconfig')
 
 def add_build_opts(parser) -> None:

tools/testing/kunit/kunit_kernel.py

···
 			return
 
 		if kunitconfig_path:
+			if os.path.isdir(kunitconfig_path):
+				kunitconfig_path = os.path.join(kunitconfig_path, KUNITCONFIG_PATH)
 			if not os.path.exists(kunitconfig_path):
 				raise ConfigError(f'Specified kunitconfig ({kunitconfig_path}) does not exist')
 		else:

tools/testing/kunit/kunit_tool_test.py

···
 		with tempfile.NamedTemporaryFile('wt') as kunitconfig:
 			tree = kunit_kernel.LinuxSourceTree('', kunitconfig_path=kunitconfig.name)
 
+	def test_dir_kunitconfig(self):
+		with tempfile.TemporaryDirectory('') as dir:
+			with open(os.path.join(dir, '.kunitconfig'), 'w') as f:
+				pass
+			tree = kunit_kernel.LinuxSourceTree('', kunitconfig_path=dir)
+
 	# TODO: add more test cases.