X test that shows problem of SliceByRootRev where untracked blocks are not added uniformly into whole history

=== RUN   TestΔFtailSliceXXX
2021/08/05 18:07:35 zodb: FIXME: open /tmp/TestΔFtailSliceXXX2265944622/001/1.fs: raw cache is not ready for invalidations -> NoCache forced
    δftail_test.go:689: slice (@at0,@at2]:
        have: [@at1·{0 1}S @at2·{1}S]
        want: [@at1·{0 1}S @at2·{0 1}S]