catapult: introduce HGTESTCATAPULTSERVERPIPE to control run-tests' tracing If unset, it will inherit its value from HGCATAPULTSERVERPIPE (which hg itself also respects). By setting only HGTESTCATAPULTSERVERPIPE, we can get per-command breakdowns of the test runtimes for the whole test suite without overloading the trace file with the contents of the tracing from hg (such as demandimport, etc.) Differential Revision: https://phab.mercurial-scm.org/D5216