test: Update coverage tests to Go 1.20

Go 1.20 finally includes proper support for instrumenting binaries for
coverage. This allows us to drop quite a few hacks and workarounds that
we used for it, and we can now also test exiting cases.

The downside is that coverage tests now require Go 1.20, but it is an
acceptable price to pay for the more accurate results.

Normal integration tests are unchanged.

This patch updates the coverage testing infrastructure to make use of
the new Go 1.20 features.

chasquid.go

@@ -235,7 +235,7 @@ func signalHandler() {
 	var err error
 
 	signals := make(chan os.Signal, 1)
-	signal.Notify(signals, syscall.SIGHUP)
+	signal.Notify(signals, syscall.SIGHUP, syscall.SIGTERM, syscall.SIGINT)
 
 	for {
 		switch sig := <-signals; sig {
@@ -251,6 +251,8 @@ func signalHandler() {
 			if err != nil {
 				log.Fatalf("Error reopening maillog: %v", err)
 			}
+		case syscall.SIGTERM, syscall.SIGINT:
+			log.Fatalf("Got signal to exit: %v", sig)
 		default:
 			log.Errorf("Unexpected signal %v", sig)
 		}