testutils: remove internal ABI comment

Some testutils external symbols are marked semi-internals, other
internals. It is not obvious what guarantees these symbols provide.
However, tests are now installable, and require the ABI stability
that glib provides for the rest of the symbols.

In my case, I would like to introduce g_assert* compat code for older
glib versions, and be able to use the so-called "internal" ABI.

I propose this change to the headers comments to explain the stability
guarantees. Removing the "internal" = you must not use this, in favor
of semi-internal = this is not documented.

https://bugzilla.gnome.org/show_bug.cgi?id=756077