selftests: Hook more tests into the build infrastructure

We have some dmabuf-heaps and perf_events tests but they are not hooked
up to the kselftest build infrastructure which is a bit of an obstacle
to running them in systems with generic infrastructure for selftests.
Add them to the top level kselftest Makefile so they get built as
standard.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>