[libcxx] [test] Don't leave test dirs behind in fs.op.current_path on Windows

[mstorsjo]

Summary

Previously, every test run left 2 directories behind in the temp dir.

[curdeius]

LGTM.

[ldionne]

Can you use CWDGuard from libcxx/test/support/filesystem_test_helper.h instead?

[mstorsjo]

Fixed nesting of static_env and CWDGuard, restore the cwd (with CWDGuard) before cleaning up the static_env.

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

TEST_CASE(current_path_test)
     TEST_CHECK(is_directory(p));
 
     const path p2 = current_path();
     TEST_CHECK(p2 == p);
 }
 
 TEST_CASE(current_path_after_change_test)
 {
-    CWDGuard guard;
     static_test_env static_env;
+    CWDGuard guard;
     const path new_path = static_env.Dir;
     current_path(new_path);
     TEST_CHECK(current_path() == new_path);
 }
 
 TEST_CASE(current_path_is_file_test)
 {
-    CWDGuard guard;
     static_test_env static_env;
+    CWDGuard guard;
     const path p = static_env.File;
     std::error_code ec;
     const path old_p = current_path();
     current_path(p, ec);
     TEST_CHECK(ec);
     TEST_CHECK(old_p == current_path());
 }
 
 TEST_CASE(set_to_non_absolute_path)
 {
-    CWDGuard guard;
     static_test_env static_env;
+    CWDGuard guard;
     const path base = static_env.Dir;
     current_path(base);
     const path p = static_env.Dir2.filename();
     std::error_code ec;
     current_path(p, ec);
     TEST_CHECK(!ec);
     const path new_cwd = current_path();
     TEST_CHECK(new_cwd == static_env.Dir2);