Fix flaky UrlExists test

[pattonwebz]

There is a test at: /tests/phpunit/helper-functions/UrlExistsTest.php:20

It makes a request to an external site (httpbin) to retrieve an image and check the url exists. The test is flaky though and sometimes the request times out or fails.

Either remove the test; find a different, more reliable, external service; or add a retry logic.

[SteveJonesDev]

@pattonwebz, doesn't this PR fix this issue? #713

[pattonwebz]

@SteveJonesDev yeah I must have not tagged it in the PR summary.

Fixed by: #713