Improve stability of command/check test module.
Improve stability of command/check test module.

When pgbackrest was present this test behaved unexpectedly.

While the binary is not currently required for this test is might be in the future so fix the test to prevent a regression.

pgbackrest/pgbackrest,pgmasters/backrest,pgbackrest/pgbackrest,pgbackrest/pgbackrest,pgmasters/backrest