test_SeqIO_lazy_formats: catch & ignore warnings

Warnings are issued when slicing records with ref or ref_db
attributes. These warnings are now silenced in the tester.
Also, an expected warning on parsing the genprot file 1MRR_A.gp
was silenced.
SeqIO._lazy: added BiopythonExperimentalWarning

The warning was added when the lazy keyword evaluates
to true in either SeqIO.index_db or SeqIO.parse.
test_SeqIO_lazy: silence BiopythonExperimentalWarning