Treat MINGW and MSYS the same in pg_upgrade test script

On msys2, 'uname -s' reports a string starting MSYS instead on MINGW
as happens on msys1. Treat these both the same way. This reverts
608a710195 in favor of a more general solution.

Backpatch to all live branches.

src/bin/pg_upgrade/test.sh

@@ -31,11 +31,13 @@ standard_initdb() {
 	../../test/regress/pg_regress --config-auth "$PGDATA"
 }
 
-# Establish how the server will listen for connections
-testhost=`uname -s`
+# What flavor of host are we on?
+# Treat MINGW* (msys1) and MSYS* (msys2) the same.
+testhost=`uname -s | sed s/^MSYS/MINGW/`
 
+# Establish how the server will listen for connections
 case $testhost in
-	MINGW*|MSYS*)
+	MINGW*)
 		LISTEN_ADDRESSES="localhost"
 		PGHOST=localhost
 		;;