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.