tools: memory-hotplug fix unexpected operator error

on-off-test uses "$UID != 0" to test for root, but $UID is a construct
specific to bash.  Using /bin/sh that isn't bash results in the
following error (due to the "$UID" part expanding to nothing):

  ./on-off-test.sh: 9: [: !=: unexpected operator

Change Makefile to use bash instead.
Signed-off-by: NShuah Khan <shuah.kh@samsung.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/memory-hotplug/Makefile

 all:
 
 run_tests:
-	@/bin/sh ./on-off-test.sh || echo "memory-hotplug selftests: [FAIL]"
+	@/bin/bash ./on-off-test.sh || echo "memory-hotplug selftests: [FAIL]"
 
 clean: