Switch the memory store's failure under its lock, so a test cannot race the keeper

The keeper keeps each transition from its own goroutine, which reads the memory
store's Fail field under the store's lock; tests assigned the exported field
bare, so TestAnUnreadableStoreClearsNothing failed under -race whenever the
goroutine appended in that window. The field is now set only through SetFail
(and Told's likewise), and a test makes the race certain rather than rare.
Test-only: the controller runs the bus store, never InMemory.

cmd/mesh-controller/conditions_test.go

@@ -86,7 +86,7 @@ func TestASilenceThroughTheVerbHoldsAndTheConditionStaysOpen(t *testing.T) {
 func TestUnreadableConditionsAreNotAWellMesh(t *testing.T) {
 	open := aMesh(t)
 	_, store := withConditionsInMemory(t)
-	store.Fail = errors.New("the bus is away")
+	store.SetFail(errors.New("the bus is away"))
 	asked, err := theThreeQuestions(t.Context(), open)
 	if err != nil {
 		t.Fatal(err)
@@ -98,7 +98,7 @@ func TestUnreadableConditionsAreNotAWellMesh(t *testing.T) {
 	if !strings.HasPrefix(said, "the open conditions could NOT be read") || strings.Contains(said, "no open conditions") {
 		t.Fatalf("%s", said)
 	}
-	store.Fail = nil
+	store.SetFail(nil)
 	asked, _ = theThreeQuestions(t.Context(), open)
 	said = printed(t, func() error { return printStatus(asked) })
 	if asked.well() && !strings.Contains(said, "no open conditions;") {