The Singularity container platform doesn't change automatically to the mounted stage directory

[rsarm]

In the Singularity container platform the change to the mounted stage directory is done with

reframe/reframe/core/containers.py

Lines 257 to 258 in 0d2c112

	if self.workdir:
	run_opts.append(f'-W {self.workdir}')

but -W doesn't change the directory like docker or sarus. From singularity --help:

  -W, --workdir string         working directory to be used for /tmp,
                               /var/tmp and $HOME (if -c/--contain was
                               also used)

As a result the test doesn't start in the stage directory.