A new test for missing image files referenced from CSS I'm adding in https://gerrit.wikimedia.org/r/#/c/235903/ exposed an issue in MobileFrontend:
```
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:
<snip>
3) ResourcesTest::testFileExistence with data set #1366 ('/var/www/html/w/extensions/MobileFrontend/resources/mobile.pag
esummary.styles/noimage.png', 'mobile.pagesummary.styles', '/var/www/html/w/extensions/MobileFrontend/resources/mobile.p
agesummary.styles/noimage.png')
File '/var/www/html/w/extensions/MobileFrontend/resources/mobile.pagesummary.styles/noimage.png' referenced by 'mobile.p
agesummary.styles' must exist.
Failed asserting that file "/var/www/html/w/extensions/MobileFrontend/resources/mobile.pagesummary.styles/noimage.png" e
xists.
/var/www/html/w/tests/phpunit/structure/ResourcesTest.php:24
/var/www/html/w/tests/phpunit/MediaWikiTestCase.php:131
<snip>
FAILURES!
Tests: 1847, Assertions: 5901, Failures: 4.
```
(This is only visible if you fix T111516 first.)
The "noimage" looks confusing, but there is indeed a "noimage.svg" file in that directory that is missing a "noimage.png" counterpart, while the styles use both. noimage.png was separated from its brother in d1f1d1b74ef1f270c78b62c29367dcf1993e825b and deleted in 9c0456c7aa891d629cd8d766c037017a585f8a88.