Don't assume expr is available in pgbench tests

Windows hosts do not normally come with expr, so instead of using that
to test the \setshell command, use echo instead, which is fairly
universally available.

Backpatch to release 11, where this came in.

Problem found by me, patch by Fabien Coelho.

Author: adunstan

src/bin/pgbench/t/001_pgbench_with_server.pl

@@ -510,7 +510,7 @@ sub pgbench
 		qr{processed: 1/1},
 		qr{shell-echo-output}
 	],
-	[qr{command=8.: int 2\b}],
+	[qr{command=8.: int 1\b}],
 	'pgbench backslash commands',
 	{
 		'001_pgbench_backslash_commands' => q{-- run set
@@ -522,10 +522,10 @@ sub pgbench
 \sleep 0 s
 \sleep :zero
 -- setshell and continuation
-\setshell two\
-  expr \
-    1 + :one
-\set n debug(:two)
+\setshell another_one\
+  echo \
+    :one
+\set n debug(:another_one)
 -- shell
 \shell echo shell-echo-output
 }