isolationtester: append session name to application_name.

isolationtester: append session name to application_name.

When writing / debugging an isolation test it sometimes is useful to see which
session holds what lock etc. To make it easier, both as part of spec files and
interactively, append the session name to application_name. Since b1907d688
application_name already contains the test name, this appends the session's
name to that.

insert-conflict-specconflict did something like this manually, which can now
be removed.

As we have done lately with other test infrastructure improvements, backpatch
this change, to make it easier to backpatch tests.

Author: Andres Freund 
Reviewed-By: Michael Paquier 
Reviewed-By: Andrew Dunstan 
Discussion: https://postgr.es/m/20211211012052[email protected]
Backpatch: 10-, to make backpatching of tests easier.