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

Tests in internal/ircserver and internal/db failed under heavy host load and passed on a rerun.

testTimeout, the failure limit on every read and wait in internal/ircserver, goes from 5 to 30 seconds; passing tests take no longer. TestQuietClientNotAnsweringPingIsClosed gets its own 4-second ping interval and measures its upper bound from the end of registration, a second before a two-interval read deadline would close the connection, leaving room for scheduling delay. endlessQueryTimeout in internal/db goes from 100 ms to 2 seconds, so the cancelled-query tests interrupt a running statement.

Judgement call: the closed test no longer checks PING timing; TestQuietClientAnsweringPingStaysConnected still does.

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