Fixed intermittent test errors

Fixes #1124

- Updated default mocha timeout to 15 seconds. Any future tests that
depend on a timeout (and thus might be better off with a lower value)
can override the default setting which is now 15 seconds.
- Removed test-specific timeout overrides for the mocha tests.
- Fixed the editor/splitbutton tests to wait for the appropriate
selector instead of assuming the dom has been updated immediately.
Should resolve intermittent timeouts when checking the splitbutton's
applied classes.