Bug 1453381 - Add docs for running Marionette xpcshell tests. r=whimboo

This duplicates documentation under testing/marionette/doc, but
can be useful when inspecting the directory.

MozReview-Commit-ID: 7vuUze1SFib

--HG--
extra : rebase_source : 94f2f4de916e576deffe1b3a23beff392b7bb594

testing/marionette/test/unit/README.md

@@ -0,0 +1,14 @@
+To run the tests in this directory, from the top source directory,
+either invoke the test despatcher in mach:
+
+	% ./mach test testing/marionette/test/unit
+
+Or call out the harness specifically:
+
+	% ./mach xpcshell-test testing/marionette/test/unit
+
+The latter gives you the `--sequential` option which can be useful
+when debugging to prevent tests from running in parallel.
+
+When adding new tests you must make sure they are listed in
+xpcshell.ini, otherwise they will not run on try.