Add a LESS test suite

Add the LessFileCompilationTest test case class, which represents the
validation of a LESS file by compilation.

Add the LessTestSuite test suite, which tests all LESS files registered
with the ResourceLoader, and use this to rewrite the checkLess.php
maintenance script.

Bug: 54665
Change-Id: Iedb8dc31e4817d8b4e40b655cf9b8fb092979e90
Merge "Add a LESS test suite"