Sanitized paths for CRAM tests in Windows mix `/` and `\`

Expected Behaviour

Path sanitization, such as the root of the test directory should work seamlessly while using CRAM tests on Unixes and Windows.

Actual Behaviour

Sanitization of paths such as the root of the test directory does not work on Windows due to a mix of / and \.
For example, if the following line is added to the only test in Dune that runs on Windows:

   $ echo $BUILD_PATH_PREFIX_MAP

The result will be:

   $ echo $BUILD_PATH_PREFIX_MAP
+ $TESTCASE_ROOT=C%.\OCaml64\home\u\dune\_build/.sandbox/b3ef.../default/test/blackbox-tests/test-cases/windows-diff.t:...
  

Starting with the _build directory path separators are slashes instead of backslashes.

Reproduction

1. Install an OCaml Cygwyn environement using fdopen's opam repository
2. Clone dune master and edit the test file as shown above.
3. Run the tests.

Specifications

• Version of dune (output of dune --version): 2.7.0-380-g08d27d547
• Version of ocaml (output of ocamlc --version)
• Operating system (distribution and version): 4.1..1

Start by locating the CRAM test harness and the path-sanitization code that produces BUILD_PATH_PREFIX_MAP, then reproduce the issue with the Windows test and inspect the generated $TESTCASE_ROOT. Done means sanitized paths use consistent separators on Windows and Unix, with the relevant CRAM test passing on Windows.