It is a undefined behavior in gles spec, but we should generate an error.

This change also refactored the coding style for shader in the test.

BUG=angleproject:2260

Change-Id: I7b480e8b66486d9954f7c7f6e8683298e94b6ad7
Reviewed-on: https://chromium-review.googlesource.com/764797Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>