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/gocovcat/gocovcat.go

@@ -0,0 +1,92 @@
+//usr/bin/env go run "$0" "$@"; exit $?
+//
+// From: https://git.lukeshu.com/go/cmd/gocovcat/
+//
+//go:build !coverage
+// +build !coverage
+
+// Copyright 2017 Luke Shumaker <lukeshu@parabola.nu>
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+// Command gocovcat combines multiple go cover runs, and prints the
+// result on stdout.
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+func handleErr(err error) {
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		os.Exit(1)
+	}
+}
+
+func main() {
+	modeBool := false
+	blocks := map[string]int{}
+	for _, filename := range os.Args[1:] {
+		file, err := os.Open(filename)
+		handleErr(err)
+		buf := bufio.NewScanner(file)
+		for buf.Scan() {
+			line := buf.Text()
+
+			if strings.HasPrefix(line, "mode: ") {
+				m := strings.TrimPrefix(line, "mode: ")
+				switch m {
+				case "set":
+					modeBool = true
+				case "count", "atomic":
+					// do nothing
+				default:
+					fmt.Fprintf(os.Stderr, "Unrecognized mode: %s\n", m)
+					os.Exit(1)
+				}
+			} else {
+				sp := strings.LastIndexByte(line, ' ')
+				block := line[:sp]
+				cntStr := line[sp+1:]
+				cnt, err := strconv.Atoi(cntStr)
+				handleErr(err)
+				blocks[block] += cnt
+			}
+		}
+		handleErr(buf.Err())
+	}
+	keys := make([]string, 0, len(blocks))
+	for key := range blocks {
+		keys = append(keys, key)
+	}
+	sort.Strings(keys)
+	modeStr := "count"
+	if modeBool {
+		modeStr = "set"
+	}
+	fmt.Printf("mode: %s\n", modeStr)
+	for _, block := range keys {
+		cnt := blocks[block]
+		if modeBool && cnt > 1 {
+			cnt = 1
+		}
+		fmt.Printf("%s %d\n", block, cnt)
+	}
+}