For a given absolute or relative path, measure the size of the file.
This is useful for measuring if the cache has increased as expected
after moving a camera or decreased after cleaning up.
In a more hackish manner, could be used for collecting binary size
statistics of shared libraries.
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`.
