tests: restrict the GL_ABGR_EXT format to the types in the spec

GL_UNSIGNED_INT_2_10_10_10_REV, GL_UNSIGNED_INT_10_10_10_2,
GL_UNSIGNED_SHORT_1_5_5_5_REV and GL_UNSIGNED_SHORT_5_5_5_1 types
are not explicitly allowed to work with GL_ABGR_EXT format neither
in the GL spec nor GL_EXT_abgr spec.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>