Apply more mechanical simplifications using recent Go features

This is a follow-up on the `go fix` run in commit
0210c5fd92.

That commit recorded the automated updates.

This one applies further simplifications that are mechanical and do not
involve logic changes, but are outside of what `go fix` did.

It also removes some now-obsolete comments and statements.

test/util/conngen/conngen.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // SMTP connection generator, for testing purposes.
 package main
 
@@ -8,7 +6,6 @@ import (
 	"net"
 	"net/http"
 	"net/smtp"
-	"time"
 
 	_ "net/http/pprof"
 
@@ -66,9 +63,7 @@ func main() {
 	log.Infof("done, created %d simultaneous connections", *count)
 
 	if *wait {
-		for {
-			time.Sleep(24 * time.Hour)
-		}
+		select {}
 	}
 
 	for _, c := range conns {

test/util/coverhtml/coverhtml.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // Generate an HTML visualization of a Go coverage profile.
 // Serves a similar purpose to "go tool cover -html", but has a different
 // visual style.

test/util/fexp/fexp.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // Fetch an URL, and check if the response matches what we expect.
 //
 // Useful for testing HTTP(s) servers.
@@ -11,10 +9,11 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"maps"
 	"net/http"
 	"os"
 	"regexp"
-	"sort"
+	"slices"
 	"strconv"
 	"strings"
 )
@@ -85,12 +84,7 @@ func main() {
 		fmt.Printf("Request: %s\n", url)
 		fmt.Printf("Response:\n")
 		fmt.Printf("  %v  %v\n", resp.Proto, resp.Status)
-		ks := []string{}
-		for k := range resp.Header {
-			ks = append(ks, k)
-		}
-		sort.Strings(ks)
-		for _, k := range ks {
+		for _, k := range slices.Sorted(maps.Keys(resp.Header)) {
 			fmt.Printf("  %v: %s\n", k,
 				strings.Join(resp.Header[k], ", "))
 		}

test/util/generate_cert/generate_cert.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // Utility to generate self-signed certificates.
 // It generates a self-signed x509 certificate and key pair, and writes them
 // to "fullchain.pem" and "privkey.pem".

test/util/loadgen/loadgen.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // SMTP load generator, for testing purposes.
 package main
 
@@ -114,9 +112,7 @@ func main() {
 	)
 
 	if *wait {
-		for {
-			time.Sleep(24 * time.Hour)
-		}
+		select {}
 	}
 }
 

test/util/minidns/minidns.go

@@ -1,5 +1,3 @@
-//go:build !coverage
-
 // minidns is a trivial DNS server used for testing.
 //
 // It takes an "answers" file which contains lines with the following format: