perl prove tests in t need updating for PG15

[df7cb]

PG 15 doesn't ship TestLib.pm anymore (renamed to PostgreSQL::Test::Util):

13:29:05 Can't locate TestLib.pm in @INC (you may need to install the TestLib module) (@INC contains: /usr/lib/postgresql/15/lib/pgxs/src/makefiles/../../src/test/perl /etc/perl /usr/local/lib/x86_64-linux-gnu/perl/5.36.0 /usr/local/share/perl/5.36.0 /usr/lib/x86_64-linux-gnu/perl5/5.36 /usr/share/perl5 /usr/lib/x86_64-linux-gnu/perl-base /usr/lib/x86_64-linux-gnu/perl/5.36 /usr/share/perl/5.36 /usr/local/lib/site_perl) at t/010_pglogical_create_subscriber.pl line 5.
13:29:05 BEGIN failed--compilation aborted at t/010_pglogical_create_subscriber.pl line 5.
13:29:05 t/010_pglogical_create_subscriber.pl .. 

Spotted by the Debian regression tests.

https://pgdgbuild.dus.dg-i.net/job/pglogical-binaries/180/architecture=amd64,distribution=sid/console#console-section-2