perf test: Test pipe mode with data conversion --to-json

perf test: Test pipe mode with data conversion --to-json

Add pipe mode test for json data conversion. Tidy up exit and cleanup
code.

Committer testing:

  $ perf test 'perf data convert --to-json'
  124: 'perf data convert --to-json' command test                      : Ok
  $ perf test -vv 'perf data convert --to-json'
  124: 'perf data convert --to-json' command test:
  --- start ---
  test child forked, pid 548738
  Testing Perf Data Conversion Command to JSON
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.020 MB /tmp/__perf_test.perf.data.krxvl (104 samples) ]
  [ perf data convert: Converted '/tmp/__perf_test.perf.data.krxvl' into JSON data '/tmp/__perf_test.output.json.0z60p' ]
  [ perf data convert: Converted and wrote 0.075 MB (104 samples) ]
  Perf Data Converter Command to JSON [SUCCESS]
  Validating Perf Data Converted JSON file
  The file contains valid JSON format [SUCCESS]
  Testing Perf Data Conversion Command to JSON (Pipe mode)
  [ perf record: Woken up 2 times to write data ]
  [ perf record: Captured and wrote 0.046 MB - ]
  [ perf data convert: Converted '-' into JSON data '/tmp/__perf_test.output.json.0z60p' ]
  [ perf data convert: Converted and wrote 0.081 MB (110 samples) ]
  Perf Data Converter Command to JSON (Pipe mode) [SUCCESS]
  Validating Perf Data Converted JSON file
  The file contains valid JSON format [SUCCESS]
  ---- end(0) ----
  124: 'perf data convert --to-json' command test                      : Ok
  $

Signed-off-by: Ian Rogers <irogers@google.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: Derek Foreman <derek.foreman@collabora.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>