all: Remove lib/util package (#9049)

Grab-bag packages are nasty, this cleans it up a little by splitting it
into topical packages sempahore, netutil, stringutil, structutil.

lib/testutil/testutil.go

@@ -0,0 +1,61 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package testutil
+
+import (
+	"errors"
+	"sync"
+)
+
+var ErrClosed = errors.New("closed")
+
+// BlockingRW implements io.Reader, Writer and Closer, but only returns when closed
+type BlockingRW struct {
+	c         chan struct{}
+	closeOnce sync.Once
+}
+
+func NewBlockingRW() *BlockingRW {
+	return &BlockingRW{
+		c:         make(chan struct{}),
+		closeOnce: sync.Once{},
+	}
+}
+
+func (rw *BlockingRW) Read(_ []byte) (int, error) {
+	<-rw.c
+	return 0, ErrClosed
+}
+
+func (rw *BlockingRW) Write(_ []byte) (int, error) {
+	<-rw.c
+	return 0, ErrClosed
+}
+
+func (rw *BlockingRW) Close() error {
+	rw.closeOnce.Do(func() {
+		close(rw.c)
+	})
+	return nil
+}
+
+// NoopRW implements io.Reader and Writer but never returns when called
+type NoopRW struct{}
+
+func (*NoopRW) Read(p []byte) (n int, err error) {
+	return len(p), nil
+}
+
+func (*NoopRW) Write(p []byte) (n int, err error) {
+	return len(p), nil
+}
+
+type NoopCloser struct{}
+
+func (NoopCloser) Close() error {
+	return nil
+}