perf tests: Fix thread map test for -F option

perf tests: Fix thread map test for -F option

I hit a bug when running test suite without forking each test (-F
option):

  $ perf test -Fv
  ...
  34: Test thread map                                          :
  --- start ---
  FAILED tests/thread-map.c:24 wrong comm
  ---- end ----
  Test thread map: FAILED!

The reason was the process name wasn't 'perf' as expected by the test,
because other tests set the name as well.

Setting it explicitly now.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Tested-by: Nilay Vaish <nilayvaish@gmail.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1467113345-12669-3-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>