build: Avoid testing some UI files

If UI files contain widgets from libhandy, tests seems to fail
8 jobs for wip/sadiq/various-ui-fixes in 11 minutes and 45 seconds (queued for 1 second)
Status Name Job ID Coverage
  Build
passed build-pureos-gcc-byzantium #376754
librem5

00:03:03

passed build-pureos-gcc-byzantium-no-purple #376753
librem5

00:01:45

 
  Test
passed check-po #376756
librem5

00:00:14

passed test:debian-gcc #376755
librem5

00:03:06

19.0%
 
  Package
passed package-pureos-byzantium #376757
librem5

00:01:47

passed package-pureos-byzantium:arm64 #376758
librem5:arm64

00:05:34

 
  Test Package
passed autopkgtest-pureos-byzantium-package #376759
librem5

00:01:18

passed lintian-pureos-byzantium-package #376760
librem5

00:00:37