Add new env variable to specify test path

This commit adds the env variable OS_TEST_PATH to specify the test
discovery path for testr. This can be used to specify which path will
be used for test discovery.
Closes-Bug: #1260710

Change-Id: Idcc8f6b5d82a2adc1a1f8ad04a47e8d694dc7de7
Merge "Add new env variable to specify test path"