test_config: Fix failure and segfault when config_hook is run twice.

On second run the config_hook test was unexpectedly failing to load
test_config.conf because it was still unmodified since the last load.
This is fixed by not passing CONFIG_FLAG_FILEUNCHANGED for the initial
loads, only using it when we are tested that a reload of unmodified
files do not initiate the hook.

ASTERISK-25960

Change-Id: Ifd679509a23ed163e5cc647490bf7df4ae3cd856