fs/tmp: add forgotten test suite

fs/tmp_posix.cpp

@@ -18,7 +18,6 @@ cruft::fs::mktmpdir (void)
 {
     // The pattern suffix we'll append to the temp directory.
     //
-    // * It needs a slash to ensure we don't try to create a new directory.
     // * We might as well append the package name for clarity
     // * The libc call REQUIRES the string end with "XXXXXX"
     char const SUFFIX[] = "/" PACKAGE_NAME "-XXXXXX";
@@ -32,5 +31,5 @@ cruft::fs::mktmpdir (void)
     if (!mkdtemp (path.data ()))
         cruft::posix::error::throw_code ();
 
-    return path;
+    return std::filesystem::path (path);
 }