Bug 1637737 - cleanup test failures from `mach run` JS shell changes;...

Bug 1637737 - cleanup test failures from `mach run` JS shell changes; r=firefox-build-system-reviewers,rstewart

In bug 1637665, we changed the OS X codepath for `mach run` to consult
`MOZ_WIDGET_TOOLKIT` as a substitute for "browserness".  Unfortunately,
this caused tests that only set `MOZ_MACBUNDLE_NAME` to fail.  Rather
than set `MOZ_WIDGET_TOOLKIT` in those tests, we probably should have
been checking `MOZ_MACBUNDLE_NAME` in the first place, since
`MOZ_MACBUNDLE_NAME` is also set only for the browser and not for the JS
shell.  We can also remove the xulrunner codepath, as xulrunner was
removed from the tree several years ago.

Differential Revision: https://phabricator.services.mozilla.com/D75315