end2end tests: add regression test for GL_LINE_LOOP drawing This was fixed by 349636a05a3577a127adb6c79a1e947890bbe462. Adding a regression test which I didn't have ready at the time. Bug: angleproject:7102 Change-Id: Icbb685c1ffad6725a278e5ed65f881f86c5688ef Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3527604 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Amirali Abdolrashidi <abdolrashidi@google.com> Commit-Queue: Charlie Lao <cclao@google.com>