Bug 1772393 - Allow 'mach jstests' in --disable-tests builds r=iain,sfink

Similar to 'mach jit-tests', we now can allow 'mach jstests' to work with just
the normal build artifacts.

Differential Revision: https://phabricator.services.mozilla.com/D148152

testing/mach_commands.py

@@ -582,6 +582,7 @@ def executable_name(name):
     "jstests",
     category="testing",
     description="Run SpiderMonkey JS tests in the JS shell.",
+    ok_if_tests_disabled=True,
 )
 @CommandArgument("--shell", help="The shell to be used")
 @CommandArgument(