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.

client/example_keys_test.go

@@ -0,0 +1 @@
+../tests/integration/client/examples/example_keys_test.go

client/main_test.go

@@ -0,0 +1,37 @@
+// Copyright 2017 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package client_test
+
+import (
+	"net/http"
+	"testing"
+
+	"go.etcd.io/etcd/v3/pkg/testutil"
+)
+
+var exampleEndpoints []string
+var exampleTransport *http.Transport
+
+func forUnitTestsRunInMockedContext(mocking func(), example func()) {
+	mocking()
+	// TODO: Call 'example' when mocking() provides realistic mocking of transport.
+
+	// The real testing logic of examples gets executed
+	// as part of ./tests/integration/client/example/...
+}
+
+func TestMain(m *testing.M) {
+	testutil.MustTestMainWithLeakDetection(m)
+}