tooling: test-js-with-puppeteer: Accept full relative file path.
tooling: test-js-with-puppeteer: Accept full relative file path.

Changes in a529dc8 to raise exception for invalid file name
has removed support for passing full file paths.
This commit fixes it.

Thanks to Steve Howell (showell) for reporting this.

rht/zulip,kou/zulip,andersk/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,rht/zulip,kou/zulip,kou/zulip,zulip/zulip,kou/zulip,rht/zulip,zulip/zulip,kou/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip