BUILD fix: file_manager/common/js/BUILD.gn unittest_util

unittest_util is a js_library: it is not js_unittest, best I can tell.
No change in behavior, no new tests.

Bug: 905934
No-try: true
Change-Id: Ib508c7380ca7babf970895ce7d69545d219daaa6
Reviewed-on: https://chromium-review.googlesource.com/c/1345370
Commit-Queue: Noel Gordon <noel@chromium.org>
Reviewed-by: Stuart Langley <slangley@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609867}

ui/file_manager/file_manager/common/js/BUILD.gn

@@ -160,7 +160,8 @@ js_library("mock_entry") {
 js_library("progress_center_common") {
 }
 
-js_unittest("unittest_util") {
+js_library("unittest_util") {
+  testonly = true
   # Only files app tests use this util file.
   visibility = []
   visibility = [ "//ui/file_manager/file_manager/*" ]