VertexAttributeTests used 'mediump' precision to sample ushort
with 65533/65534/65535 which exceed the range defined in ESSL Spec.
(Spec "4.5.2 Precision Qualifiers")

Change precision of attribute 'test' and 'expected' to 'highp'
for numbers that exceed the minimum range.

Bug: angleproject:5211
Change-Id: I01e5c999ec43d17a7390835d2ac88ce2bc0b4c5b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2491621Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>