flatpak: Use g_assert_true() in self tests

g_assert() can be accidentally compiled out with G_DISABLE_ASSERT;
g_assert_true() cannot.
2 jobs for flatpak-self-test-runtimerepo in 4 minutes and 6 seconds
Status Job ID Name Coverage
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passed #830167
debian-stable-x86_64

00:02:26

passed #830166
fedora-x86_64

00:04:06