selftests/Makefile: make `run_tests' depend on `all'

So a "make run_tests" will build the tests before trying to run them.

Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>