A new test for missing image files referenced from CSS I'm adding in https://gerrit.wikimedia.org/r/#/c/235903/ exposed an issue:
```
b@ubuntu15:/var/www/wiki/tests/phpunit$ php phpunit.php structure/ResourcesTest.php
PHPUnit already present
PHPUnit 3.7.37 by Sebastian Bergmann.
Configuration read from /var/www/html/w/tests/phpunit/suite.xml
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..........................FF................................. 488 / 1847 ( 26%)
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........................FF................................... 1403 / 1847 ( 75%)
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.................
Time: 32.96 seconds, Memory: 42.50Mb
There were 4 failures:
1) ResourcesTest::testFileExistence with data set #453 ('/var/www/html/w/mw-config/images/help-question.gif', 'mediawiki
.legacy.config', '/var/www/html/w/mw-config/images/help-question.gif')
File '/var/www/html/w/mw-config/images/help-question.gif' referenced by 'mediawiki.legacy.config' must exist.
Failed asserting that file "/var/www/html/w/mw-config/images/help-question.gif" exists.
/var/www/html/w/tests/phpunit/structure/ResourcesTest.php:24
/var/www/html/w/tests/phpunit/MediaWikiTestCase.php:131
2) ResourcesTest::testFileExistence with data set #454 ('/var/www/html/w/mw-config/images/help-question-hover.gif', 'med
iawiki.legacy.config', '/var/www/html/w/mw-config/images/help-question-hover.gif')
File '/var/www/html/w/mw-config/images/help-question-hover.gif' referenced by 'mediawiki.legacy.config' must exist.
Failed asserting that file "/var/www/html/w/mw-config/images/help-question-hover.gif" exists.
/var/www/html/w/tests/phpunit/structure/ResourcesTest.php:24
/var/www/html/w/tests/phpunit/MediaWikiTestCase.php:131
<snip>
/var/www/html/w/tests/phpunit/structure/ResourcesTest.php:24
/var/www/html/w/tests/phpunit/MediaWikiTestCase.php:131
FAILURES!
Tests: 1847, Assertions: 5901, Failures: 4.
```
This appears to be a bug in 1e06a9f02f9a3ca645f5abe5cd38943fcfa3aea2 where we forgot to actually copy the two image files when copying CSS…