[libcxx] [test] Fix the fs.op.absolute test to cope with windows paths

[mstorsjo]

Summary

Prepend the root path on the already_absolute testcase, and construct a path ending with the preferred separator for the test reference for "foo/".

Additionally, the "" case fails both on MS STL and libstdc++, where it returns an empty string (and libstdc++ sets the error code). Unsure what to do with that one - make it libcpp specific, or consider the other implementations being wrong in thas aspect?

libcxx/test/std/input.output/filesystems/fs.op.funcs/fs.op.absolute/absolute.pass.cpp

 {
     const fs::path cwd = fs::current_path();
     const struct {
       std::string input;
       fs::path expect;
     } TestCases [] = {
         {"", cwd / ""},
         {"foo", cwd / "foo"},
-        {"foo/", cwd / "foo/"},
-        {"/already_absolute", "/already_absolute"}
+        {"foo/", cwd / "foo" / ""},
+        {"/already_absolute", cwd.root_path() / "already_absolute"}
     };
     for (auto& TC : TestCases) {
         std::error_code ec = GetTestEC();
         const path ret = absolute(TC.input, ec);
         TEST_CHECK(!ec);
         TEST_CHECK(ret.is_absolute());
         TEST_CHECK(PathEq(ret, TC.expect));
     }
 }
 
 TEST_SUITE_END()