Simplify output tests with expectOutputString()

[birgire]

PHPUnit has methods to test output, e.g.:

expectOutputString()
expectOutputRegex()

​https://phpunit.de/manual/current/en/writing-tests-for-phpunit.html#writing-tests-for-phpunit.output

We could use them to simplify tests.

Example: Before:

ob_start();
the_post_thumbnail();
$actual = ob_get_clean();
$this->assertEquals( '', $actual );

After:

$this->expectOutputString( '' );
the_post_thumbnail();

Currently used in files:

tests/phpunit/tests/general/template.php
tests/phpunit/tests/general/document-title.php
tests/phpunit/tests/general/resourceHints.php
tests/phpunit/tests/oembed/WpEmbed.php

We could scope this ticket to e.g. the files:

tests/phpunit/tests/post/thumbnails.php
tests/phpunit/tests/post/listPages.php
tests/phpunit/tests/user/listAuthors.php
tests/phpunit/tests/functions/getArchives.php
tests/phpunit/tests/taxonomy.php

to begin with.

[birgire]

In 42098.diff​ simplifies output tests in:

tests/phpunit/tests/post/thumbnails.php
tests/phpunit/tests/post/listPages.php
tests/phpunit/tests/user/listAuthors.php
tests/phpunit/tests/functions/getArchives.php
tests/phpunit/tests/taxonomy.php

[johnbillion]

In 42382:

Build/Test tools: Switch to PHPUnit's expectOutputString() method instead of manually using output buffers for output assertions.

Props birgire
Fixes #42098