X wcfs: assert that keycov only grow

Currently fails with e.g.:

(neo) (z-dev) (g.env) kirr@deca:~/src/neo/src/lab.nexedi.com/nexedi/wendelin.core/wcfs/internal/xbtree$ go test -short -v -failfast -run 'TestΔBTail$/Update/T3/B1:a-B8:c→T7/B1:a-B8:c'
=== RUN   TestΔBTail
2021/06/23 14:08:45 zodb: FIXME: open /tmp/δBTail926507346/1.fs: raw cache is not ready for invalidations -> NoCache forced
=== RUN   TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c
    δbtail_test.go:812: BUG: tkeyCov1 ⊄ tkeyCov2:
                tkeyCov1: {[3,∞)}
                tkeyCov2: {[7,∞)}
                tkeyCov1 \ tkeyCov2: {[3,7)}
    δbtail_test.go:708: T3/B1:a-B8:c -> T7/B1:a-B8:c  ; tracked={8} :

        d12:  map[]
        δTok: map[]
        δT:   map[]

        δZ:               {000000000000000c}
        Tracked^δZ:       {}
        kadj[Tracked^δZ]: {}
        kadj: map[1:{1 8 9223372036854775807} 8:{1 8 9223372036854775807} 9223372036854775807:{1 8 9223372036854775807}]

--- FAIL: TestΔBTail (0.84s)
    --- FAIL: TestΔBTail/Update/T3/B1:a-B8:c→T7/B1:a-B8:c (0.00s)

wcfs/internal/xbtree/δbtail_test.go

@@ -806,6 +806,12 @@ func xverifyΔBTail_Update1(t *testing.T, subj string, db *zodb.DB, treeRoot zod
 	δbtail_ := δbtail.Clone()
 	δB1, err := δbtail_._Update1(δZ);  X(err)	// XXX don't compute treediff twice
 
+	// assert tkeyCov1 ⊂ tkeyCov2
+	x := tkeyCov1.Difference(tkeyCov2)
+	if !x.Empty() {
+		t.Errorf("BUG: tkeyCov1 ⊄ tkeyCov2:\n\ttkeyCov1: %s\n\ttkeyCov2: %s\n\ttkeyCov1 \\ tkeyCov2: %s", tkeyCov1, tkeyCov2, x)
+	}
+
 	// assert δtkeycov == δ(tkeyCov1, tkeyCov2)
 	δtkeycovOK := tkeyCov2.Difference(tkeyCov1)
 	//fmt.Printf("tkeyCov1: %s\n", tkeyCov1)