tests/opr/softsig-t: Avoid hanging due to intermediate sh -c

tests/opr/softsig-t: Avoid hanging due to intermediate sh -c

If the build directory happened to contain shell metacharacters, like
the ~ in /build/openafs-vb8tid/openafs-1.8.0~pre1 used by the Debian
builders, Perl was running softsig-helper via an intermediate sh -c,
which would then intercept the signals we tried to send to
softsig-helper.  Use the list syntax to avoid this sh -c.

Change-Id: I054b9c8f606e197accb414bfe3f89719255c62c4
Reviewed-on: https://gerrit.openafs.org/12488
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>