Synchronize libregrtest from master to 3.6 (#2244)

* bpo-30523: regrtest: Add --list-cases option (#2238)
* bpo-30284: Fix regrtest for out of tree build (#1481)
* bpo-30540: regrtest: add --matchfile option (#1909)
* bpo-30258: regrtest: Fix run_tests_multiprocess() (#1479)
* bpo-30263: regrtest: log system load (#1452)
7 files changed