Adds --remap-path-prefix as an unstable option. This is implemented to
mimic the behavior of rustc's --remap-path-prefix but with minor
adjustments.
This flag similarly takes in two paths, a prefix to replace and a
replacement string.
apparently I missed some tests in the last commit. Rather than having
dozens of tests use the long version, use the short version in
`run-make` and the long version in `run-make-fulldeps` (which is now
only three tests)