build: Valgrind script now depends on unit test executable

build: Valgrind script now depends on unit test executable

This prevents Sporadic Valgrind Failures, caused by race conditions
Common error message is like:
  out/linux/x86_64/debug/resource/unittests/unittests: Permission denied

Bug: https://jira.iotivity.org/browse/IOT-1526
Change-Id: I49bfb21ee7cd934b965e132cb150d59c047ef383
Signed-off-by: Philippe Coval <philippe.coval@osg.samsung.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/14705
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Kevin Kane <kkane@microsoft.com>
Reviewed-by: Nivedita Singhvi <niveditasinghvi@gmail.com>
Reviewed-by: Ziran Sun <ziran.sun@samsung.com>