Move testsuite 'genload' function to its own file.

Move testsuite 'genload' function to its own file.

* testsuite/lib/genload.exp: Move code from systemtap.stress/current.exp
  to its own file so it can be used multiple places.
* testsuite/config/unix.exp: Load genload.exp.
* testsuite/systemtap.stress/current.exp: Move 'genload' function to its
  own file.
* testsuite/systemtap.stress/all_kernel_functions.exp: Use 'genload'
  function (instead of having a copy).