Use TESTS_ENVIRONMENT rather than AM_TESTS_ENVIRONMENT

Change-Id: I7f0f5e7b76d6a508227f5d3afd9ce5c570871dc0
Reviewed-on: http://review.couchbase.org/28225
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>