scanner: Fix get_symbols/comments to maintain the scanner lists

Use g_slist_copy prior to returning the lists run through g_slist_reverse.
This preserves the source scanners internally held lists where previously
they would only point to a single element after a call, leaking memory and
breaking subsequent calls. Note the functions as (transfer container) and
use g_slist_free after calls in the Python bindings.
Add new unittest file: test_sourcescanner.py for isolated unittesting of the
SourceScanner.

https://bugzilla.gnome.org/show_bug.cgi?id=581525