testCMFActivity: support pickle protocol > 0 in test_insert_max_payload

This test assumed that activating a method with a string of length `n+x`
as argument would be serialized in `x` more bytes that activating the
same method with a string of length `n` as argument, which is only true
for protocol 0 for multiple reasons ( short strings and long strings
are serialized differently, "frames" are used with protocol 5) and
db.string_literal seems to introduce a difference.

Change the test to mock Message.dump to produce longer dumps instead of
relying on this assumption.