Update FileLoader to fix issue #10339

FileLoader now uses resolved FileLoader's (the one that explicitly
supports that resource) FileLocatorInterface instance before falling
back to its own when trying to load resources in import() method.
added a BC comment
changed some PHPUnit assertions to more specific ones
Merge branch '2.3' into 2.4

* 2.3:
  changed some PHPUnit assertions to more specific ones
  fixed Kernel::stripComments() normalizing new-lines
  added a BC comment
  Update FileLoader to fix issue #10339