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.

internal/conditions/memory_test.go

@@ -0,0 +1,30 @@
+package conditions
+
+import (
+	"errors"
+	"fmt"
+	"testing"
+)
+
+// **A test may take the store away while the keeper is still telling.** The keeper keeps every
+// transition from a goroutine of its own, so the memory store's failure is read there while a test
+// switches it: it is switched under the store's lock, or the race detector fails the suite at random
+// (it once failed TestAnUnreadableStoreClearsNothing). A hundred transitions still being kept while
+// the failure is switched a hundred times makes the race certain, not rare, when the lock is skipped.
+func TestTheStoreIsTakenAwayWhileTheKeeperIsTelling(t *testing.T) {
+	k, store, told, _ := keeper(t)
+	ctx := t.Context()
+	const n = 100
+	for i := range n {
+		if _, err := k.Observe(ctx, silent(fmt.Sprintf("m%d", i))); err != nil {
+			t.Fatal(err)
+		}
+	}
+	for range n {
+		store.SetFail(errors.New("the bus is away"))
+		store.SetFail(nil)
+	}
+	told.SetFail(errors.New("no responders"))
+	told.SetFail(nil)
+	settled(t, told, n)
+}