MAX_DRAW_BUFFERS has a minimum value of 4.  This test was
unconditionally using 6 draw buffers.

Bug: angleproject:4092
Bug: angleproject:4180
Change-Id: I069874a6402da1cb007bf9504e68b9910951cf63
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1944527Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>