perf test: Implement skip_reason callback for watchpoint tests

perf test: Implement skip_reason callback for watchpoint tests

Currently reason for skipping the read only watchpoint test is only seen
when running in verbose mode:

  $ perf test watchpoint
  23: Watchpoint                                            :
  23.1: Read Only Watchpoint                                : Skip
  23.2: Write Only Watchpoint                               : Ok
  23.3: Read / Write Watchpoint                             : Ok
  23.4: Modify Watchpoint                                   : Ok

  $ perf test -v watchpoint
  23: Watchpoint                                            :
  23.1: Read Only Watchpoint                                :
  --- start ---
  test child forked, pid 60204
  Hardware does not support read only watchpoints.
  test child finished with -2

Implement skip_reason callback for the watchpoint tests, so that it's
easy to see reason why the test is skipped:

  $ perf test watchpoint
  23: Watchpoint                                            :
  23.1: Read Only Watchpoint                                : Skip (missing hardware support)
  23.2: Write Only Watchpoint                               : Ok
  23.3: Read / Write Watchpoint                             : Ok
  23.4: Modify Watchpoint                                   : Ok

Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20201016131650.72476-1-tommi.t.rantala@nokia.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>