Test should clear before drawing.  With the clears added it became
apparent the second draw wasn't working because an attrib was left in
instanced mode after the first draw.

BUG=angleproject:2672

Change-Id: I6c3ee7302d24babb80a9b9affcf357145bb03dcb
Reviewed-on: https://chromium-review.googlesource.com/c/1306822Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>