Let ircserver and db tests pass under heavy host load (closes #265)

testTimeout, the limit on every read and wait in the ircserver tests,
goes from 5 to 30 seconds. A passing test never reaches it.

TestQuietClientNotAnsweringPingIsClosed uses a 3 second ping interval,
and measures its upper bound from the end of registration. That leaves
2.7 seconds for scheduling delay before the close a two-interval read
deadline would make. The lower bound, from the client's last line, is
unchanged.

The cancelled-query tests in internal/db run the endless query for 2
seconds instead of 100 ms. A context that ends before the statement
starts interrupts nothing, so the pool kept its connection.

Model: opus-5-5

internal/ircserver/server_test.go

@@ -557,14 +557,13 @@ func TestQuietClientNotAnsweringPingIsClosed(t *testing.T) {
 	t.Parallel()
 
 	// A read deadline of two ping intervals would close the
-	// connection a second after latest. The ping interval is
+	// connection at latest. The ping interval is longer than
-	// longer than testPingInterval, so that the time between
+	// testPingInterval, so that the time between the expected
-	// the expected close and latest leaves room for a loaded
+	// close and latest leaves room for a loaded host.
-	// host.
 	const (
-		interval = 4 * time.Second
+		interval = 3 * time.Second
 		earliest = interval + testPongDeadline
-		latest   = 2*interval - time.Second
+		latest   = 2 * interval
 	)
 
 	env := newTestEnvWithPingTimes(