Relax whitespace sensitivity in Filecheck

[sunfishcode]

Filecheck currently has some whitespace sensitivities that are inconvenient when writing tests.

This issue was originally reported in https://github.com/Cretonne/cretonne/issues/54; see that issue for further discussion.

[lars-t-hansen]

Piling on: The test directive "; nextln:" has different meaning from "; nextln: ".