tests: use debugdeltachain where appropriate
Some tests are verifying delta chain type things. This metadata
has more to do with a revlog implementation details than index
data, which is theoretically generic.
This commit ports some tests to `hg debugdeltachain`, as it is the
more appropriate debug command for looking at delta metadata.
Differential Revision: https://phab.mercurial-scm.org/D3026