test: close fds properly in tests

Otherwise the unclosed fd may keep the file open which makes
subsequent rmdir attempts failed at exit.

test/parallel/test-fs-writefile-with-fd.js

@@ -66,6 +66,7 @@ tmpdir.refresh();
 
   fs.open(file, 'r', common.mustSucceed((fd) => {
     fs.writeFile(fd, 'World', common.expectsError(expectedError));
+    fs.closeSync(fd);
   }));
 }
 
@@ -79,6 +80,7 @@ tmpdir.refresh();
     fs.writeFile(fd, 'World', { signal }, common.expectsError({
       name: 'AbortError'
     }));
+    fs.closeSync(fd);
   }));
 
   controller.abort();