[clang][deps] Simplify PREFIX definitions in tests

Instead of outputting the test directory into the JSON result file, parsing it with `FileCheck` and then potentially stripping it, simply use `FileCheck`'s `-D` option.

Note that we use `%/t` instead of `%t` in order to normalize to forward slashes on Windows, which matches what we do with `sed 's:\\\\\?:/:g'`.

Differential Revision: https://reviews.llvm.org/D121516