ewaol-tests/oci-containers: Integration test runner improvements

This patch provides three improvements:
* Allow standalone test runner to be executed from any directory.
* Adjust conditional check for BATS success to check return status rather than
  output.
* If the BATS variables $output and $status are null or unset, they now
  simply default to empty string, rather than emit an error.

Issue-Id: SCM-2363
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I71da12353e5ce019441f85728112e200b9ec1d27