pgsql: Don't assume expr is available in pgbench tests

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.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1a721248f3899ccf8c0c7512b91d8458f2394aeb

Modified Files
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src/bin/pgbench/t/001_pgbench_with_server.pl | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)