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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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/1357181d46249d211837e1df7a49ee1131027a99

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