Remove incidental md5() function uses from several tests

Remove incidental md5() function uses from several tests

This 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.  This will eventually allow these tests to
pass in OpenSSL FIPS mode (which does not allow MD5 use).  See also
208bf364a9.  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.

Reviewed-by: Daniel Gustafsson <[email protected]>
Discussion: https://www.postgresql.org/message-id/flat/f9b480b5-e473-d2d1-223a-4b9db30a229a@eisentraut.org