tests/helper-runtests.sh

rust: Add a Timestamp struct instead of abusing Duration

`SystemTime` would be the standard library type semantically appropriate
instead of `Duration`.

But since the value is coming from Python as a plain integer and used in
dirstate packing code as an integer, let’s make a type that contains a single
integer instead of using one with sub-second precision.

Differential Revision: https://phab.mercurial-scm.org/D10485

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL