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>

tools/testing/selftests/Makefile

@@ -133,4 +133,4 @@ clean:
 		make OUTPUT=$$BUILD_TARGET -C $$TARGET clean;\
 	done;
 
-.PHONY: install
+.PHONY: all run_tests hotplug run_hotplug clean_hotplug run_pstore_crash install clean