test: enable sparse-revlog for test-stream-bundle-v2.t
We are about to enable sparse-revlog globally. To help with reviewing the
tests change, we isolate them in individual changesets.
Differential Revision: https://phab.mercurial-scm.org/D5339
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# Avoid interference from actual test env:
unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL