selftests: move watchdog tests from Documentation/watchdog

Remove watchdog-test from Makefile to move the test to selftests.

Add Makefile and .gitignore for watchdog-test. watchdog-test will
not be run as part of selftests suite and will not be included in
install targets.  It can be built separately for now.

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>