tests: use exec to call libwrap'd executables

No need to leave the shell process hanging around.

In particular, if we are manually running softsig-helper under
libwrap to debug test failures, the child process of the shell is
another shell, which interprets some signals that we wanted to
be passed through, like SIGTERM.  On the other hand, once the
softsig-helper is exec()'d, you basically need another shell to
terminate it, which is a different problem....

Change-Id: Iff7c519886a018cb68e692746d40c427b6299457
Reviewed-on: https://gerrit.openafs.org/12490
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Anders Kaseorg <andersk@mit.edu>
Tested-by: Anders Kaseorg <andersk@mit.edu>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

tests/libwrap

@@ -45,4 +45,4 @@ fi
 export LIBPATH
 
 shift
-"$TEST_COMMAND" "$@"
+exec "$TEST_COMMAND" "$@"