kunit: add ability to specify suite-level init and exit functions

KUnit has support for setup/cleanup logic for each test case in a suite.
But it lacks the ability to specify setup/cleanup for the entire suite
itself.

This can be used to do setup that is too expensive or cumbersome to do
for each test.
Or it can be used to do simpler things like log debug information after
the suite completes.
It's a fairly common feature, so the lack of it is noticeable.

Some examples in other frameworks and languages:
* https://docs.python.org/3/library/unittest.html#setupclass-and-teardownclass
* https://google.github.io/googletest/reference/testing.html#Test::SetUpTestSuite

Meta:
This is very similar to this patch here: https://lore.kernel.org/linux-kselftest/20210805043503.20252-3-bvanassche@acm.org/
The changes from that patch:
* pass in `struct kunit *` so users can do stuff like
  `kunit_info(suite, "debug message")`
* makes sure the init failure is bubbled up as a failure
* updates kunit-example-test.c to use a suite init
* Updates kunit/usage.rst to mention the new support
* some minor cosmetic things
  * use `suite_{init,exit}` instead of `{init/exit}_suite`
  * make suite init error message more consistent w/ test init
  * etc.

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/kunit-example-test.c

@@ -40,6 +40,17 @@ static int example_test_init(struct kunit *test)
 	return 0;
 }
 
+/*
+ * This is run once before all test cases in the suite.
+ * See the comment on example_test_suite for more information.
+ */
+static int example_test_init_suite(struct kunit_suite *suite)
+{
+	kunit_info(suite, "initializing suite\n");
+
+	return 0;
+}
+
 /*
  * This test should always be skipped.
  */
@@ -142,17 +153,20 @@ static struct kunit_case example_test_cases[] = {
  * may be specified which runs after every test case and can be used to for
  * cleanup. For clarity, running tests in a test suite would behave as follows:
  *
+ * suite.suite_init(suite);
  * suite.init(test);
  * suite.test_case[0](test);
  * suite.exit(test);
  * suite.init(test);
  * suite.test_case[1](test);
  * suite.exit(test);
+ * suite.suite_exit(suite);
  * ...;
  */
 static struct kunit_suite example_test_suite = {
 	.name = "example",
 	.init = example_test_init,
+	.suite_init = example_test_init_suite,
 	.test_cases = example_test_cases,
 };