Make the second merge check pass: the gate's mesh has a chain of its own; the tests follow delivery 2's meaning (issues 1121, 1108, 1109)

The merge gate raises the snapshot's mesh in a throwaway store and now gives it a chain in memory, since the chain lives only on the bus. A module with no commit is carried by no send (a send is read from the commits on the assignments' keys), so the grants step flags only a module with a commit as newly assigned. The tests that sent a module with no build record one, the merge test waits for the merges' consumer before publishing and for the last look of a window before the next test, the pending test follows a chain that deletes nothing, and a wait's settling is a value of its key.

internal/inventory/chain.go

@@ -71,6 +71,14 @@ func (i *Inventory) UseChain(c *chain.Chain) {
 	}
 }
 
+// UseThrowawayChain gives the inventory a chain in memory that is its own, laid out already: for a mesh raised for a moment
+// beside the real one, as the merge gate raises the facts snapshot's mesh in a throwaway store (issue 1121). Nothing of it
+// reaches the bus.
+func (i *Inventory) UseThrowawayChain() {
+	i.UseChain(chain.InMemory(nil))
+	i.markLaidOut()
+}
+
 // attached is the chain this inventory has, nil when none.
 func (i *Inventory) attached() *chain.Chain {
 	i.state.mu.Lock()