test: use g_assert_not_reached () instead of asserting FALSE

test: use g_assert_not_reached () instead of asserting FALSE

It's declared as never returning, making do a better at understanding control
flow. Otherwise it makes a poor guess:

  In file included from test-crypto.c:37:
  ../../shared/nm-test-utils.h:1344:3: error: variable 'family' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
                  g_assert (FALSE);
                  ^~~~~~~~~~~~~~~~