resourceloader: CSSMin::getLocalFileReferences now strips anchors

resourceloader: CSSMin::getLocalFileReferences now strips anchors

When processing:

  url( 'foo.svg#anchors' )

getLocalFileReferences() would yield a file 'foo.svg#anchors'. It might
occur with upstream libraries, such as WikiFonts used by the Refreshed
skin.

When a path is found, strip the anchor entirely. Skip the case if that
results in an empty file.

remap() would properly skip the behavior "#default#', also skip url that
are solely an anchor such as '#other'.

Add a few test cases to cover CSSMin::getLocalFileReferences and cover
anchors usage in CSSMin::remap.

Bug: T115436
Change-Id: I1749ddc2b89021807f42d64131931ad7a99a7b43