Add iio-test-gts to mod-internal.list

In the past few weeks while building linux-next for my kernel vanilla repositories I noticed a few upstream changes that require changes to files like mod-internal.list or filter-modules.sh.fedora for kernel-ark builds to succeed. Changes like these will most likely be needed in kernel-ark/os-build soon, as the those upstream changes are heading towards mainline for 6.9 currently afaics. I'm providing these changes here in advance in case they might helpful for anyone (either for cherry-picking for for merging as a whole).

Signed-off-by: Thorsten Leemhuis fedora@leemhuis.info