fix(test): stop asserting a cache hit against a one-second deadline

TestCacheTTLExpiry set a 1s TTL and immediately asserted a hit, so it
depended on an upper bound of elapsed wall-clock time between Set and
Get. Nothing can promise that: on the capacity-1 runner, with the rest
of the suite running in parallel, the goroutine can be descheduled for
longer than the TTL and the entry is then correctly gone.

It failed that way on this PR while passing five times out of five
locally, and it touches no code this branch changed.

Two entries now: one with an hour to live carries the presence
assertions, one with a second carries the expiry. Sleeping past a TTL
is always safe, so only the direction that cannot flake is timed.

backend/explore/cache_test.go

@@ -46,23 +46,42 @@ func TestCacheMiss(t *testing.T) {
 	}
 }
 
+// TestCacheTTLExpiry checks both halves of the TTL contract, and uses two
+// entries to do it.
+//
+// **No assertion here may depend on an upper bound of elapsed wall-clock
+// time**, which is what the single-entry version of this test did: it set
+// a 1s TTL and immediately asserted a *hit*, so on a loaded runner — one
+// goroutine descheduled for over a second while the rest of the suite
+// runs — the entry was correctly gone and the test failed with "expected
+// cache hit immediately after set".  It did exactly that in CI while
+// passing five times out of five locally.
+//
+// Sleeping *past* a TTL is always safe, so the expiry half keeps a short
+// one; the presence half gets a TTL nothing can outrun.
 func TestCacheTTLExpiry(t *testing.T) {
 	c := newTestCache(t)
 
 	data := []byte(`{"ephemeral":true}`)
-	c.Set("ttl-test-key", data, 1*time.Second, "", "")
+	c.Set("ttl-live-key", data, time.Hour, "", "")
+	c.Set("ttl-expiring-key", data, 1*time.Second, "", "")
 
-	// Verify it's there immediately.
-	if _, ok := c.Get("ttl-test-key"); !ok {
-		t.Fatal("expected cache hit immediately after set")
+	if _, ok := c.Get("ttl-live-key"); !ok {
+		t.Fatal("expected a cache hit on an entry with an hour to live")
 	}
 
-	// Wait for expiry.
+	// Wait for the short one to expire.
 	time.Sleep(2 * time.Second)
 
-	if _, ok := c.Get("ttl-test-key"); ok {
+	if _, ok := c.Get("ttl-expiring-key"); ok {
 		t.Error("expected cache miss after TTL expiry, got hit")
 	}
+
+	// And the long-lived entry is still there, which is what says the
+	// sweep above expired an entry rather than the cache.
+	if _, ok := c.Get("ttl-live-key"); !ok {
+		t.Error("the hour-long entry expired too")
+	}
 }
 
 func TestCacheMBID(t *testing.T) {