tests/: add test suite for stackprof

tests/: add test suite for stackprof

Together with the stackprof tool we add some test cases to exercise
it, and the underlying libdwfl_stacktrace functionality as well.

* tests/stackprof-subr.sh: Common subroutines for checking
  perf_events availability and verifying gmon.*.out files.
* tests/run-stackprof-system.sh: New file.
* tests/run-stackprof-system-gprof.sh: New file.
* tests/run-stackprof-user.sh: New file.
* tests/run-stackprof-user-gprof.sh: New file.
* tests/Makefile.am (TESTS): add the new testcases.

Co-authored-by: <fche@redhat.com>
Signed-off-by: <serhei@serhei.io>