tests/helper-runtests.sh

dirstate-tree: Add map `get` and `contains_key` methods

Differential Revision: https://phab.mercurial-scm.org/D10369

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# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL