tests: Make examples (for not client) to be both: documentation and integration-runnable

This CL tries to connect 2 objectives:
  - Examples should be close (the same package) to the original code,
    such that they can participate in documentation.
  - Examples should be runnable - such that they are not getting out of
    sync with underlying API/implementation.

In case of etcd-client, the examples are assuming running 'integration'
style, i.e. thay do connect to fully functional etcd-server.
That would lead to a cyclic dependencies between modules:
  - server depends on client (as client need to be lightweight)
  - client (for test purposes) depend on server.
Go modules does not allow to distingush testing dependency from
prod-code dependency.

Thus to meet the objective:
  - The examples are getting executed within testing/integration packages against real etcd
  - The examples are symlinked to 'unit' tests, such that they included in documentation.
  - Long-term the unit examples should get rewritten to use 'mocks' instead of real integration tests.

pkg/testutil/leak.go

@@ -130,18 +130,23 @@ func interestingGoroutines() (gs []string) {
 	return gs
 }
 
-// MustTestMainWithLeakDetection expands standard m.Run with leaked
-// goroutines detection.
-func MustTestMainWithLeakDetection(m *testing.M) {
-	v := m.Run()
-
+func MustCheckLeakedGoroutine() {
 	http.DefaultTransport.(*http.Transport).CloseIdleConnections()
 
 	// Let the other goroutines finalize.
 	runtime.Gosched()
 
-	if v == 0 && CheckLeakedGoroutine() {
+	if CheckLeakedGoroutine() {
 		os.Exit(1)
 	}
+}
+
+// MustTestMainWithLeakDetection expands standard m.Run with leaked
+// goroutines detection.
+func MustTestMainWithLeakDetection(m *testing.M) {
+	v := m.Run()
+	if v == 0 {
+		MustCheckLeakedGoroutine()
+	}
 	os.Exit(v)
 }