lockdep/selftest: Add wait context selftests

These tests are added for two purposes:

*	Test the implementation of wait context checks and related
	annotations.

*	Semi-document the rules for wait context nesting when
	PROVE_RAW_LOCK_NESTING=y.

The test cases are only avaible for PROVE_RAW_LOCK_NESTING=y, as wait
context checking makes more sense for that configuration.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201208103112.2838119-5-boqun.feng@gmail.com