test: Improve layout of helper binaries

This patch moves the test helper binaries to a "one directory per
helper" layout, and also makes them to be ignored in the coverage build
instead of all builds.

With this change, "go build ./..." will build all binaries including the
test helpers, which helps make sure that module manage automation also
considers them. In particular, this makes "go mod tidy" work fine.

test/util/conngen/conngen.go

@@ -0,0 +1,110 @@
+//go:build !coverage
+// +build !coverage
+
+// SMTP connection generator, for testing purposes.
+package main
+
+import (
+	"flag"
+	"net"
+	"net/http"
+	"net/smtp"
+	"time"
+
+	"golang.org/x/net/trace"
+
+	_ "net/http/pprof"
+
+	"blitiri.com.ar/go/log"
+)
+
+var (
+	addr = flag.String("addr", "",
+		"server address")
+	httpAddr = flag.String("http_addr", "localhost:8011",
+		"monitoring HTTP server listening address")
+	wait = flag.Bool("wait", false,
+		"don't exit after --run_for has lapsed")
+	count = flag.Int("count", 1000,
+		"how many connections to open")
+)
+
+var (
+	host string
+	exit bool
+)
+
+func main() {
+	var err error
+
+	flag.Parse()
+	log.Init()
+
+	host, _, err = net.SplitHostPort(*addr)
+	if err != nil {
+		log.Fatalf("failed to split --addr=%q: %v", *addr, err)
+	}
+
+	if *wait {
+		go http.ListenAndServe(*httpAddr, nil)
+		log.Infof("monitoring address: http://%v/debug/requests?fam=one&b=11",
+			*httpAddr)
+	}
+
+	log.Infof("creating %d simultaneous connections", *count)
+	conns := []*C{}
+	for i := 0; i < *count; i++ {
+		c, err := newC()
+		if err != nil {
+			log.Fatalf("failed to connect #%d: %v", i, err)
+		}
+
+		conns = append(conns, c)
+
+		if i%200 == 0 {
+			log.Infof("  ... %d connections", i)
+
+		}
+	}
+	log.Infof("done, created %d simultaneous connections", *count)
+
+	if *wait {
+		for {
+			time.Sleep(24 * time.Hour)
+		}
+	}
+}
+
+type C struct {
+	tr trace.Trace
+	n  net.Conn
+	s  *smtp.Client
+}
+
+func newC() (*C, error) {
+	tr := trace.New("conn", *addr)
+
+	conn, err := net.Dial("tcp", *addr)
+	if err != nil {
+		return nil, err
+	}
+
+	client, err := smtp.NewClient(conn, host)
+	if err != nil {
+		conn.Close()
+		return nil, err
+	}
+
+	err = client.Hello(host)
+	if err != nil {
+		return nil, err
+	}
+
+	return &C{tr: tr, n: conn, s: client}, nil
+}
+
+func (c *C) close() {
+	c.tr.Finish()
+	c.s.Close()
+	c.n.Close()
+}