use PLUGIN_COMMAND_PREFIX instead of SERVICE for command prefix

tests/test_helper.bash

@@ -1,12 +1,11 @@
 #!/usr/bin/env bash
-
 export DOKKU_QUIET_OUTPUT=1
 export DOKKU_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/dokku"
 export DOKKU_VERSION=${DOKKU_VERSION:-"master"}
 export PATH="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/bin:$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/dokku:$PATH"
+export PLUGIN_COMMAND_PREFIX="redis"
 export PLUGIN_PATH="$DOKKU_ROOT/plugins"
 export PLUGIN_DATA_ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)/fixtures"
-export SERVICE=redis
 
 mkdir -p $PLUGIN_DATA_ROOT