selftests/capabilities: actually test it

The capatabilities exist in selftest but no in the TARGETS list.
Add it to the TARGETS.
Signed-off-by: NBamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: NShuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/Makefile

 TARGETS = breakpoints
+TARGETS += capabilities
 TARGETS += cpu-hotplug
 TARGETS += efivarfs
 TARGETS += exec