scripts/resulttool: limit the number of changes displayed per test

scripts/resulttool: limit the number of changes displayed per test

Most of the changes list generated in regression reports fall in one
of the two following categories:
- there is only a few (<10) changes listed and the info is
  valuable/relevant
- the list is huge (> 100 ? 1000 ?) and basically tells us that the whole
  tests category suffers the same status (test missing, test failing, test
  skipped, etc)

Prevent those huge, worthless lists by limiting the output for each test
group:
- current default limit is arbitrarily set to 50
- limit can still be overriden with a new "-l"/"--limit" flag, either with
  custom value, or with 0 to print the whole lists of changes
- limit is applied per test family: currently it distinguishes only types
  of ptests, but it can be adapted to other kind of tests

Signed-off-by: Alexis Lothoré <alexis.lothore@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>