tests: bump default timeout to 360s A number of tests hit or almost hit the default limit even on modern hardware. While the tests are ideally split into smaller pieces, that's non-trivial work. HyperThreading and similar technologies can trigger this often, even without any other load on the machine. Differential Revision: https://phab.mercurial-scm.org/D10534

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# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL