modetest: Avoid erroring if there's no gamma legacy support

Let's follow the Rule of Silence. And while here,
document what's going on.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
6 jobs for gamma-fixes in 2 minutes and 42 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Containers
passed debian #2304310

00:00:20

 
  Build
passed meson-aarch64 #2304313

00:00:26

passed meson-armhf #2304314

00:00:27

passed meson-i386 #2304312

00:00:25

passed meson-ppc64el #2304315

00:02:19

passed meson-x86_64 #2304311

00:00:25