test: fix flaky test-https-client-get-url

Fixed test-https-client-get-url by waiting on HTTPS GET requests
to finish before closing the server.

PR-URL: https://github.com/nodejs/node/pull/12876
Fixes: https://github.com/nodejs/node/issues/12873
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

test/parallel/test-https-client-get-url.js

@@ -41,18 +41,21 @@ const options = {
   cert: fs.readFileSync(`${common.fixturesDir}/keys/agent1-cert.pem`)
 };
 
-const server = https.createServer(options, common.mustCall(function(req, res) {
+const server = https.createServer(options, common.mustCall((req, res) => {
   assert.strictEqual('GET', req.method);
   assert.strictEqual('/foo?bar', req.url);
   res.writeHead(200, {'Content-Type': 'text/plain'});
   res.write('hello\n');
   res.end();
-  server.close();
 }, 3));
 
-server.listen(0, function() {
+server.listen(0, common.mustCall(() => {
-  const u = `https://127.0.0.1:${this.address().port}/foo?bar`;
+  const u = `https://${common.localhostIPv4}:${server.address().port}/foo?bar`;
-  https.get(u);
+  https.get(u, common.mustCall(() => {
-  https.get(url.parse(u));
+    https.get(url.parse(u), common.mustCall(() => {
-  https.get(new URL(u));
+      https.get(new URL(u), common.mustCall(() => {
-});
+        server.close();
+      }));
+    }));
+  }));
+}));