Test the newest merge's order, a second reopening's gaps and sub-second merge times (hq issues 348, 349)

The review of PR 179 found four paths no test held: which of two earlier
plans NewestMergeOf takes, a tie between them, gaps kept across a second
reopening in one keeper, and a merge time's fraction of a second.

internal/conditions/store_test.go

@@ -459,3 +459,38 @@ func TestAFaultKeepsItsNewestGapsAndForgetsWhatWasBefore(t *testing.T) {
 		t.Fatalf("a fault reopened at its own first raising: %+v", d)
 	}
 }
+
+// Two reopenings in one keeper, each after ClearSaying: both gaps kept, the fault begun at its first raising,
+// and open again at the second clearing's reopening.
+func TestASecondReopeningKeepsBothGaps(t *testing.T) {
+	k, _, _, c := keeper(t)
+	ctx := t.Context()
+	first, err := k.Observe(ctx, silent("ace"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var cleared []time.Time
+	for i := 0; i < 2; i++ {
+		c.pass(30 * time.Second)
+		cleared = append(cleared, c.now().UTC())
+		if ok, err := k.ClearSaying(ctx, "machine.ace.silent", "heard again", "ace is heard again"); err != nil || !ok {
+			t.Fatalf("cleared %v: %v", ok, err)
+		}
+		c.pass(time.Minute)
+		if _, err := k.Observe(ctx, silent("ace")); err != nil {
+			t.Fatal(err)
+		}
+	}
+	got, err := k.Open(ctx)
+	if err != nil || len(got) != 1 {
+		t.Fatalf("%+v %v", got, err)
+	}
+	g := got[0]
+	if !g.Began().Equal(first.Raised) || len(g.Gaps) != 2 || !g.Gaps[0].Cleared.Equal(cleared[0]) ||
+		!g.Gaps[1].Cleared.Equal(cleared[1]) || !g.Gaps[1].Reopened.Equal(g.Raised) || g.Count != 3 {
+		t.Fatalf("after two reopenings: began %s, gaps %+v, count %d", g.Began(), g.Gaps, g.Count)
+	}
+	if g.OpenAt(cleared[0].Add(time.Second)) || !g.OpenAt(cleared[0].Add(-time.Second)) || g.OpenAt(cleared[1].Add(time.Second)) {
+		t.Fatalf("open at the wrong moments: %+v", g)
+	}
+}