Fix test none/tests/pth_2sig on Solaris.

Fix test none/tests/pth_2sig on Solaris.
When signal SIGINT was used, it got propagated to the parent shell
and therefore the regression test suite was terminated.
With SIGTERM only the parent exits, as is supposed.
Verified that this changed test case still reproduces the original problem
on Linux and Solaris, when r16140 is temporarily undone.
Follow up to r16140 and r16143 and BZ#372600.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16147