Raise a check's store without durability and remove what earlier holders left (hq issue 306)

On the control node the store-bound packages of the controller's suite ran
five to seven times slower than on any other holder, and every controller
check that landed there ran past the suite's thirty minutes: a throwaway
store flushing to a disk the mesh's own store, bus and forge keep busy.
A store that lives for one check needs no crash safety.

A holder recreated mid-check left the check's store and bus running, and
the redelivery went to another machine, so nothing removed them: eleven
pairs across four machines. A starting holder has taken nothing, so every
container labelled with an ask of the seat is an earlier holder's.

cmd/mesh-builder/holder_test.go

@@ -3,6 +3,7 @@ package main
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"strings"
 	"testing"
 
@@ -146,3 +147,26 @@ func TestAKillArrivingAfterTheBuildEndedIsRefusedAndAnUnannouncedKillSaysSo(t *t
 		t.Fatalf("kill said %q (%v)", said, err)
 	}
 }
+
+// **Issue 306**: a holder starting removes the containers earlier builds left on its machine, through
+// the runner a kill uses, and a runtime that cannot be asked does not stop it.
+func TestAHolderStartingRemovesWhatEarlierBuildsLeftHere(t *testing.T) {
+	h := newHolder("novox", link.TheBuildMachine, t.TempDir(), nil)
+	var removed []string
+	h.remove = func(_ context.Context, _ string, name string, args ...string) (string, error) {
+		removed = append(removed, name+" "+strings.Join(args, " "))
+		if args[0] == "ps" {
+			return "c1 build-1\nc2 build-1\nc3 check-here-2\n", nil
+		}
+		return "", nil
+	}
+	if n := h.removeLeftBehind(); n != 2 || len(removed) != 2 || removed[1] != "docker rm -f c1 c2" {
+		t.Fatalf("removed %d: %v", n, removed)
+	}
+	h.remove = func(context.Context, string, string, ...string) (string, error) {
+		return "", errors.New("no runtime")
+	}
+	if n := h.removeLeftBehind(); n != 0 {
+		t.Errorf("a runtime that cannot be asked removed %d", n)
+	}
+}