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/db/db_test.go

@@ -26,7 +26,11 @@ const (
 		(SELECT 1 UNION ALL SELECT x + 1 FROM n)
 		SELECT COUNT(*) FROM n`
 
-	endlessQueryTimeout = 100 * time.Millisecond
+	// endlessQueryTimeout is how long the endless query runs
+	// before its context ends. A query whose context ends before
+	// it starts is never run, and the pool keeps its connection,
+	// so the timeout leaves a loaded host time to start it.
+	endlessQueryTimeout = 2 * time.Second
 )
 
 // TestDefaultDBURLSetsWALAndBusyTimeout opens a file database