tests: Replace g_assert() calls with g_assert_*() calls

https://docs.gtk.org/glib/testing.html

 warns against using g_assert()
in test cases, since it is a no-op when compiling with G_DISABLE_ASSERT.
The replacement calls also give more detailed messages on failures.

Raises the minimum required glib version for building unit tests
from 2.16 (released March 2008) to 2.40 (released March 2014) to
get support for g_assert_nonnull().

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>