[libcxx] [test] Simplify the fs helper header for posix cases. NFC.

[ldionne]

Thanks for the cleanup. LGTM once CI passes.

libcxx/test/support/filesystem_test_helper.h

inline bool space(std::string path, std::uintmax_t &capacity,
         assert(avail > 0);
         return true;
     }
 #else
     using ::mkdir;
     using ::ftruncate;
     inline int symlink(const char* oldname, const char* newname, bool is_dir) { (void)is_dir; return ::symlink(oldname, newname); }
     using ::link;
-    inline int setenv(const char *var, const char *val, int overwrite) {
-        return ::setenv(var, val, overwrite);
-    }
-    inline int unsetenv(const char *var) {
-        return ::unsetenv(var);
-    }
+    using ::setenv;
+    using ::unsetenv;
     inline bool space(std::string path, std::uintmax_t &capacity,
                       std::uintmax_t &free, std::uintmax_t &avail) {
         struct statvfs expect;
         if (::statvfs(path.c_str(), &expect) == -1)
           return false;
         assert(expect.f_bavail > 0);
         assert(expect.f_bfree > 0);
         assert(expect.f_bsize > 0);