testsuite/systemtap.base: add new testcase uprobes_filtering.exp

testsuite/systemtap.base: add new testcase uprobes_filtering.exp

This new testcase uses the newly implemented define UPROBES_HITCOUNT
(see previous commit) to verify that uprobes filtering work as it
should.

If prefiltering is supported by the kernel (from 3.9), then it verifies
that UPROBE_HANDLER_REMOVE is obeyed and that the prehandler() is not
called over and over for unwanted processes, allowing the stap script
to run at full speed.

In both cases (whether or not prefiltering is supported), it also
verifies that the probe handlers are called only for the processes of
interest and no other (i.e. it makes sure that the filtering in
stapiu_probe_prehandler() works).