tests/test-hghave.t

revbranchcache: move entry writing to a separate function

This moves the actual writing of entries to the cache to a separate function.
This will allow us to use it in multiple places. Ex: in one place we will write
dummy entries, and in another place we will write real data.

Testing that hghave does not crash when checking features

  $ "$TESTDIR/hghave" --test-features 2>/dev/null