Previously, instantiating a RspecFlaky::FlakyExample object would
automatically update its first_flaky_at, last_flaky_at and
last_flaky_job.

That was wrong because we would overwrite every time the suite report
with this false data.

We now:

- Get the suite report and only read from it
- Write only the currently detected flaky examples in the report, so
  that the final report is only updated with flaky examples that were
  actually detected in each job. Before, job1 could overwrite the legit
  report from job2!
- Write the newly detected flaky examples by rejecting the already
  tracked flaky specs instead of using another hash.
Signed-off-by: Rémy Coutable <remy@rymai.me>