selftests/Makefile: Add missed PHONY targets

`selftests/Makefile` is defining only `install` as entire PHONY target
though there are few more PHONY targets including `run_tests`.  This
commit defines them as the PHONY targets.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>