compiletest path normalization is too aggressive

[euclio]

Notice that the backslashes in the help output for intra-doc link warnings are incorrectly converted to forward slashes:

rust/src/test/rustdoc-ui/intra-links-warning.stderr

Line 8 in de5c3c4

= help: to escape `[` and `]` characters, just add '/' before them like `/[` or `/]`

The normalization is currently just a replace on all backslashes:

rust/src/tools/compiletest/src/runtest.rs

Line 3083 in b244f61

.replace("\\", "/") // normalize for paths on windows

[pnkfelix]

Could we perhaps use the canonicalize method here?

Update: I suppose that would not actually address the issue at all. We could try to make the regexp itself smarter (e.g. have it not rewrite stuff inside quotes).