squash! Modify tests for error checking on "flat" keyword to match updated specs.

In addition, I've added "check_link: true" to all the tests, since
some implementations make this check at link time.
squash! WIP: GLES3: test that non-flat varying structs cannot contain ints.
squash! Modify tests for error checking on "flat" keyword to match updated specs.

Fix version declarations.
squash! Modify tests for error checking on "flat" keyword to match updated specs.

Fix up fragment shader color stuff.