Add condition and test for explicit set of no PS1 #865

Job	Run time
eessi_container_script (listrepos_default)	1m 28s
eessi_container_script (exec)	2m 28s
eessi_container_script (container)	2m 5s
eessi_container_script (shell)	2m 40s
eessi_container_script (run)	3m 33s
eessi_container_script (no_cvmfs_mounts)	2m 33s
eessi_container_script (resume)	2m 28s
eessi_container_script (listrepos_custom)	1m 43s
eessi_container_script (help)	1m 49s
eessi_container_script (unionfs)	2m 26s
	23m 13s

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