lib: support overriding http\s.globalAgent

Overriding `require('http[s]').globalAgent` is now respected by
consequent requests.

In order to achieve that, the following changes were made:

1. Implmentation in `http`: `module.exports.globalAgent` is now defined
through `Object.defineProperty`. Its getter and setter return \ set
`require('_http_agent').globalAgent`.

2. Implementation in `https`: the https `globalAgent` is not the same
as `_http_agent`, and is defined in `https` module itself. Therefore,
the fix here was to simply use `module.exports.globalAgent` to support
mutation.

3. According tests were added for both `http` and `https`, where in
both we create a server, set the default agent to a newly created
instance and make a request to that server. We then assert that the
given instance was actually used by inspecting its sockets property.

Fixes: https://github.com/nodejs/node/issues/23281

PR-URL: https://github.com/nodejs/node/pull/25170
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/http.js

@@ -21,7 +21,7 @@
 
 'use strict';
 
-const { Agent, globalAgent } = require('_http_agent');
+const httpAgent = require('_http_agent');
 const { ClientRequest } = require('_http_client');
 const { methods } = require('_http_common');
 const { IncomingMessage } = require('_http_incoming');
@@ -52,9 +52,8 @@ module.exports = {
   _connectionListener,
   METHODS: methods.slice().sort(),
   STATUS_CODES,
-  Agent,
+  Agent: httpAgent.Agent,
   ClientRequest,
-  globalAgent,
   IncomingMessage,
   OutgoingMessage,
   Server,
@@ -76,3 +75,14 @@ Object.defineProperty(module.exports, 'maxHeaderSize', {
     return maxHeaderSize;
   }
 });
+
+Object.defineProperty(module.exports, 'globalAgent', {
+  configurable: true,
+  enumerable: true,
+  get() {
+    return httpAgent.globalAgent;
+  },
+  set(value) {
+    httpAgent.globalAgent = value;
+  }
+});