Allow MSYS as well as MINGW in Msys uname

Msys2's uname -s outputs a string beginning MSYS rather than MINGW as is
output by Msys. Allow either in pg_upgrade's test.sh.

Backpatch to all live branches.

src/bin/pg_upgrade/test.sh

@@ -34,7 +34,7 @@ standard_initdb() {
 testhost=`uname -s`
 
 case $testhost in
-	MINGW*)
+	MINGW*|MSYS*)
 		LISTEN_ADDRESSES="localhost"
 		PGHOST=localhost
 		;;