Civi\API\ExternalBatch - Improve test for variables_order/$_ENV

Civi\API\ExternalBatch - Improve test for variables_order/$_ENV

The recent revision #9595 updated `ExternalBatch` to produce a more
informative error message.  However, in the test environment used by
`flexmailer`, there was exactly 1 value in `$_ENV` (even if
`variables_order` was misconfigured).

This should make the test a bit harder to trip-up by affirmatively checking
for the most common environment variable.

Review note: In the known universe, the only direct references to
`ExternalBatch` are in `CiviUnitTestCase`, so this shouldn't impact any
runtime logic.