Make code-action-request.test and request-reply.test pass on Windows.

clangd's test:// scheme expands to /clangd-test on non-Win and to
C:/clang-test on Win, so it can't be mixed freely with
file:///clangd-test since that's wrong on Windows. This makes both
tests consistenly use the test:// scheme. (Alternatively, we could use
the //INPUT_DIR pattern used in a few other tests.)

Part of PR43592.

llvm-svn: 374746