Leave a busy container running until it says idle, within 15 minutes

A send that replaced the build agent killed the check it ran, and the queue
started it again elsewhere (issue 594). A container may now declare busy; the
node-engine asks it before replacing it, applies everything else, says the
wait, and replaces it on the first idle pass or at the bound, kept across
restarts (novox/hq ADR 0335).

cmd/mesh-host/units_test.go

@@ -87,10 +87,10 @@ func TestTheStatementSaysAnAccountJudgedForRoot(t *testing.T) {
 			Groups: []string{"openrazer"}}, State: accounts.Healthy, Since: at},
 	}}
 	h := withAccounts(healthAsReported(liveness.Statement{At: at}, nil), as)
-	// At RootContract or later: an engine that also reads an identity's runner says OwnGroupContract (novox/hq ADR
-	// 0334), one above ADR 0328's LibraryContract.
-	if h.Contract < link.RootContract || link.RootContract != 3 || h.Contract != link.OwnGroupContract ||
-		link.OwnGroupContract != link.LibraryContract+1 {
+	// At RootContract or later: an engine that also reads a container's busy says BusyContract (novox/hq ADR
+	// 0335), one above ADR 0334's OwnGroupContract.
+	if h.Contract < link.RootContract || link.RootContract != 3 || h.Contract != link.BusyContract ||
+		link.BusyContract != link.OwnGroupContract+1 {
 		t.Fatalf("contract %d", h.Contract)
 	}
 	got := map[string]string{}