Add getHttpClient() to BrowserTestBase

Problem/Motivation

Working on #2870458: Convert web tests to browser tests for quickedit module → I realized there's no direct replacement for WTB::drupalPost().

This got me searching through core for 'x-www-form-urlencoded' in order to see if I was missing something.

It turns out a number of tests make direct calls with curl, which is what WTB::drupalPost() does.

For instance here's a WTB: Drupal\node\Tests\Views\NodeContextualLinksTest::renderContextualLinks().

And here's a BTB: Drupal\Tests\history\Functional\HistoryTest::getNodeReadTimestamps().

There are many outlier WTB tests awaiting conversion to BTB, and it seems that they're outliers because there's no direct conversion for drupalPost(). And there are also many tests which are rolling their own curlExec() in order to accomplish this goal.

Proposed resolution

Make sure all tests have a consistent way to use the Mink client's guzzle client. See HistoryTest::getHttpClient() for an example of how this might work. http://cgit.drupalcode.org/drupal/tree/core/modules/history/tests/src/Fu...

Then tests can perform POST or other requests as needed using the client.

Remaining tasks

Add a change record about drupalPost(), drupalPostAjaxForm(), etc. conversions.

Follow-up to make existing tests consistent in their use of getHttpClient().

User interface changes

API changes

Data model changes