flatpak: Use g_assert_true() in self tests

g_assert() can be accidentally compiled out with G_DISABLE_ASSERT;
g_assert_true() cannot.
3 jobs for flatpak-self-test-runtimerepo in 7 minutes and 31 seconds
Status Job ID Name Coverage
  Build
passed #831239
debian-stable-x86_64

00:02:26

passed #831002
fedora-x86_64

00:05:05

failed #831003
debian-stable-x86_64

00:02:34