tools: memory-hotplug fix unexpected operator error

on-off-test is a bash script and invoked from /bin/sh
This results in the following error:

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

Changed Makefile to use bash instead.
Signed-off-by: NShuah Khan <shuah.kh@samsung.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.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: