bookmark: introduce in advance a variant of the exchange test
We are about to introduce a new way to push bookmark to server. We introduce
the test variant before actually updating the exchange to help the output
changes to stand out when it happens.
#
# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL