If G_DISABLE_ASSERT is defined, g_assert() is a no-op. Despite it now
being standard practice to *not* use g_assert() in unit tests (use
g_assert_*() instead), a lot of existing unit tests still use it.
Compiling those tests with G_DISABLE_ASSERT would make them silently
no-ops. Avoid that by warning the user loudly.

Note that it’s pretty rare for people to compile with G_DISABLE_ASSERT,
so it’s not expected that this will be hit often.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

#976