Make testSignal unittest not depend on default shell

Make testSignal unittest not depend on default shell

This patch changes the code executed when testing the signal handling
of RunCmd. Since sh does not always point to bash (e.g. on Ubuntu,
where it points to /bin/dash) this test might fail due to the returned
exit code is different so the received signal is not correctly
detected.

Additionally fix the docstring of testSignal.

(This is a backport from trunk)

Reviewed-by: iustinp

From: Manuel Franceschini <manuel.franceschini@gmail.com>