Simplify matching pattern check in TAP tests of pg_receivewal

A check in the ZLIB portion of the test to match the name of a
non-compressed partial segment with a completed compressed segment was
using m//, while a simple equality check is enough.  This makes the test
a bit stricter without impacting its coverage.

Author: Kyotaro Horiguchi
Discussion: https://postgr.es/m/20210726.174622.826565852378770261.horikyota.ntt@gmail.com

Author: michaelpq

src/bin/pg_basebackup/t/020_pg_receivewal.pl

@@ -111,7 +111,7 @@
 	# of the previous partial, now-completed WAL segment is updated, keeping
 	# its base number.
 	$partial_wals[0] =~ s/\.partial$/.gz/;
-	is($zlib_wals[0] =~ m/$partial_wals[0]/,
+	is($zlib_wals[0] eq $partial_wals[0],
 		1, "one partial WAL segment is now completed");
 	# Update the list of partial wals with the current one.
 	@partial_wals = @zlib_partial_wals;