Add map_groups__merge_in test to test the map_groups__merge_in function
usage - merging kcore maps into existing eBPF maps.

Committer testing:

  # perf test merge
  59: map_groups__merge_in                                  : Ok
  # perf test -v merge
  59: map_groups__merge_in                                  :
  --- start ---
  test child forked, pid 8349
  test child finished with 0
  ---- end ----
  map_groups__merge_in: Ok
  #
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stanislav Fomichev <sdf@google.com>
Link: http://lkml.kernel.org/r/20190508132010.14512-10-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>