perf tests: Fix PYTHONPATH for python-use test tracepoints

If there's not OUTPUT variable defined the PYTHONPATH ends up with
/python. We need to remove the extra '/'.

Signed-off-by: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/n/tip-h1hzfyfcdxjnuq9fin2cjwlr@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>