Remove incidental md5() function uses from several tests

Continuing the work started with 208bf364a9, this patch removes md5()
function calls from these test suites:

- bloom
- test_decoding
- isolation
- recovery
- subscription

This covers all remaining test suites where md5() calls were just used
to generate some random data and can be replaced by appropriately
adapted sha256() calls. Unlike for the main regression tests, I didn't
write a fipshash() wrapper here, because that would have been too
repetitive and wouldn't really save much here. In some cases it was
easier to remove one layer of indirection by changing column types from
text to bytea.