Add treeenv.go that combines Treegen and client side access to ZODB with
committed trees as extension to testing.T . The environment allows to
easily see which tree update was committed, what is the difference in
terms of KV, what is the state of updated tree and state of pointed-to
ZBlk objects.

This will be used to test upcoming ΔBtail and ΔFtail.

Main functionality is in treeenv.go; the other added files are to
support that.

Some preliminary history:

f07502fc    X xbtreetest: Teach T & Commit to automatically provide At in symbolic form
0d62b05e    X Adjust to btree.VGet & friends signature change to include keycov in visit callback
588a512a    X zdata: Switch SliceByFileRev not to clone Zinblk
e9c4b619    X rebuild: tests: Random testing
43090ac7    X tests: Factor-out tree-test-env into tTreeEnv
d4a523b2    X δbtail: tests: Run much faster with live ZODB cache
271d953d    X rebuild: tests: Move ΔBtail.Clone test out of hot inner loop into separate test
c32055fc    X wcfs/xbtree: ΔBtail tests += ø -> Tree; Tree -> ø
5324547c    X wcfs/xbtree: root(a) must stay in trackSet even after treediff(a,ø)
8f6e2b1e    X rebuild: tests: Don't access ZODB in XGetδKV