glyph-test: Avoid setting solid images as alpha maps.

glyph-test: Avoid setting solid images as alpha maps.

glyph-test would sometimes set a solid image as an alpha map, which is
not allowed. When this happened and the debug spew was enabled,
messages like this one would be generated:

    *** BUG ***
    In pixman_image_set_alpha_map: The expression
            !alpha_map || alpha_map->type == BITS was false
    Set a breakpoint on '_pixman_log_error' to debug

Fix this by not passing the ALLOW_SOLID flag to create_image() when
the resulting is to be used as an alpha map.