Fixed PR 11270 by adding nd_syscall testcase.

Fixed PR 11270 by adding nd_syscall testcase.

* 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.