Rewrite test/Linker tests to use FileCheck instead of grep.

Some translations here are not 1x1 because there are grep|grep
chains that are non-trivial to implement in terms of FileCheck features. I
made an effort for the tests to remain as similar as possible; do let me know
if you notice anything fishy. The good news are that some buggy tests were
fixed (grep | not grep - a bug waiting to happen).