Bug: Fix log file handling in test-reruns

[vsalomaki]

The rerun-PR #526 had a bug; the log files for rebot were merged in the wrong order. Effect is that the original test-results overwrote the results from the retest and the merging of reportes created a report with the original fail-results even if retest passed. As a consequence, if there was another rerun-round due to errors, the starting point was the original output.

This PR includes following:

• Correct reporting output, the output.xml, log.html and report.html contain combined information
• Add better testing so that the previous round of retest is taken into account
• Prints out retests "rounds" starting from 1 instead of 0, also suppresses (duplicate) stdout from rebot
• Update Firefox to 142. The version 141 was no longer available in the docker build
• Add reference to Rerunning tests into the Readme TOC

Screenshots from reports after, using the "Roll Dice"-tests:

[vsalomaki]

Proposing to change the recently added environment variable to a more descriptive, such as ROBOT_RERUN_MAX_ROUNDS

[vsalomaki]

Comments? ( @ppodgorsek )