GL_MAX_COLOR_ATTACHMENTS has a minimum value of 4.  The test was
unconditionally using 5 attachments.

Bug: angleproject:4172
Change-Id: I793c297b50e3fdc54f75e2c5608508279028359f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1937808
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>