Add singularity wrapper and simple test (ADCINFR-61)
- For now, only worker nodes which can access an image under CVMFS directory are valid for tests, otherwise the test returns ERROR,
- If singularity is available in both system and CVMFS, system is preferred,
- singularity-wrapper should work for other WN tests (e.g. CE-ATLAS-sft-Frontier-Squid).