See https://travis-ci.org/joomla/joomla-cms/jobs/141875477 as an example.

Any suggestions?

In my opinion most of the methods which output html, should use JLayout to support overrides. So in the tests for these methods we should only check if they call JLayout::render with the correct parameters.

Im not a friend of testing templates/layouts, because they change to often. But we could create html output tests for the important parts/elements of the layouts with DomDocument and assertEqualXMLStructure():
https://phpunit.de/manual/5.4/en/appendixes.assertions.html#appendixes.assertions.assertEqualXMLStructure

@dneukirchen for some reason assertXmlStringEqualsXmlFile() failed here: #10995
So not sure that assertEqualXMLStructure() will actually work

In that special case (we only have to check some properties/attributes and values) we could write a test for each attribute/value check and use DomDocument helpers like $this->assertTrue($actual->hasAttribute('disabled')).

Or if you wanna stay with the helper (its not so readable in my opinion. i would prefer the $this->assertTrue($actual->hasAttribute('disabled')) check i mentioned above.)

public function testGetInput($data, $expectedHtml)
{
    $formField = new JFormFieldUrl;
    foreach ($data as $attr => $value)
    {
        TestReflection::setValue($formField, $attr, $value);
    }

    $expected = new DOMDocument;
    $expected->loadXML($expectedHtml);

    $actual = new DOMDocument;
    $actual->loadXML(TestReflection::invoke($formField, 'getInput'));

    $this->assertEqualXMLStructure(
        $expected->firstChild,
        $actual->firstChild,
        true, // check attributes
        'Line:' . __LINE__ . ' The field did not produce the right html'
    );
}

In my case assertXmlStringEqualsXmlString did the job. But as soon as you add new attributes which are empty it fails. Perhaps an approach can be to add our own assertXMLStringEquals function which can compare strings and tests empty attributes.

I am not a fan of testing output in unit test, so I would simple drop them or go with David suggestion

the tests for these methods we should only check if they call JLayout::render with the correct parameters.

Agree with Robert here, we just keep fixing them again and again, because of intended output changes with really low value.

Tests are gone