[libcxx] [test] Avoid littering the temp dir with empty directories

[mstorsjo]

Summary

When the filesystem tests want to create a directory, a temporary
path is created as <temp>/<cwd filename>/static_env.0. The directory
is created, and when done, this particular directory is removed.
However, the directory <temp>/<cwd filename> is left around, empty.

(When running tests locally, the <cwd filename> bit is derived from
the individual test, like "copy_symlink.pass.cpp.dir". When doing
remote testing, the cwd name is a unique temporary name like
"libcxx.pZVQdKeSEN".)

Instead name these temporary directories as
<temp>/<cwd filename>-static_env.0. That way, when they are removed,
no extra intermediate directory is left behind.

Prior to e0d01294bc124211a8ffb55e69162eb34a242680, the temporary
directories were created in only one level, leaving no empty
directories behind.

[• Quuxplusone]

I think you should roll this into D98703 because it obviously is dependent with that one, and they're both pretty small and solve similar problems.

[mstorsjo]

Merging this with D98703.

libcxx/test/support/filesystem_test_helper.h

private:
     // This could potentially introduce a filesystem race if multiple
     // scoped_test_envs were created concurrently in the same test (hence
     // sharing the same cwd). However, it is fairly unlikely to happen as
     // we generally don't use scoped_test_env from multiple threads, so
     // this is deemed acceptable.
     static inline fs::path available_cwd_path() {
         fs::path const cwd = utils::getcwd();
         fs::path const tmp = fs::temp_directory_path();
-        fs::path const base = tmp / cwd.filename();
+        std::string base = cwd.filename().string();
         int i = 0;
-        fs::path p = base / ("static_env." + std::to_string(i));
+        fs::path p = tmp / (base + "-static_env." + std::to_string(i));
         while (utils::exists(p.string())) {
-            p = fs::path(base) / ("static_env." + std::to_string(++i));
+            p = tmp / (base + "-static_env." + std::to_string(++i));
         }
         return p;
     }
 };
 
 /// This class generates the following tree:
 ///
 ///     static_test_env