tests: actually restore the original environment before running syshg
Since os.environ may be overridden in run-tests.py, several important
variables such as PATH weren't restored.
I don't like the idea of using the system hg *by default* because the
executable and the configs are out of our control. But I don't mind as
long as the tests pass.
test-ancestor.py
test-backwards-remove.t
test-branch-tag-confict.t
test-casecollision.t
test-check-commit.t
test-check-execute.t
test-check-pyflakes.t
test-check-pylint.t
test-check-shbang.t
test-contrib-check-code.t
test-contrib-check-commit.t
test-ctxmanager.py
test-diff-issue2761.t
test-diff-reverse.t
test-diff-subdir.t
test-dirstate-nonnormalset.t
test-doctest.py
test-empty-dir.t
test-excessive-merge.t
test-issue1089.t
test-issue1993.t
test-issue842.t
test-locate.t
test-lrucachedict.py
test-manifest.py
test-merge-default.t
test-merge-subrepos.t
test-merge2.t
test-merge5.t
test-revlog-packentry.t
test-run-tests.py
test-unified-test.t
test-update-reverse.t
test-xdg.t