Before this change, the found paths to the expected images
erroneously included the path to the `metrics.json` file
(if this file was present) leading to raising of an
unhandled exception.
This change adds filter command line option to render test runner,
so that test can be filtered by providing regular expression, for instance:
mbgl-render-test -f .*hillshade.*
The `mapbox-gl-native`-specific render tests expected results will
be stored at the `render-test/expected/` subfolder.
Test runner will automatically put new expected results there if
invoked with `UPDATE=true`.
