* testsuite/systemtap.syscall/test.tcl: Now uses global variable
'test_script' to find test script to run.
* testsuite/systemtap.syscall/syscall.exp: Sets test_script.
* testsuite/systemtap.syscall/nd_sys.stp: New test script. Copy of
sys.stp, but uses nd_syscall probes.
* testsuite/systemtap.syscall/nd_syscall.exp: New testcase. Copy of
syscall.exp, but uses nd_sys.stp test script.