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
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