Add test case for obsoleting slot with active walsender

 Add test case for obsoleting slot with active walsender
 
 The code to signal a running walsender when its reserved WAL size grows
 too large is completely uncovered before this commit; this adds coverage
 for that case.
 
 This test involves sending SIGSTOP to walsender and walreceiver and
 running a checkpoint while advancing WAL, then sending SIGCONT.  There's
 no precedent for this coding in Perl tests, and my reading of relevant
 manpages says it's likely to fail on Windows.  Because of this, this
 test is always skipped on that platform.
 
 Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
 Discussion: https://postgr.es/m/202106102202.mjw4huiix7lo@alvherre.pgsql