Titles that correspond to foreign files aren't included in the set of
titles returned by ApiPageSet#getGoodTitles. However, since
ApiQueryPageImages#execute finds files with wfFindFile, these "missing"
titles can be included.
Add the ApiQueryPageImages#getTitles helper function, which returns the
set of "good" titles union the set of missing titles in the File
namespace.
Also, register the PHPUnit tests directory in the UnitTestsList hook
handler.
The original version of this patch was reverted due to a (stupid) bug
wherein a fatal error was triggered because there was no check for there
being no missing titles in the File namespace [0] and, consequently, an
invalid operand was being passed to the array union operator. This bug
is fixed as well as proven to work with a simple set of test cases.
[0] https://git.wikimedia.org/blob/mediawiki%2Fextensions%2FPageImages.git/47137e7ee671c89667d620bc04ac7649b1d9af96/ApiQueryPageImages.php#L31
Bug: T98791
Bug: T114417
Change-Id: I923e88dde3a8ced4921b4192d90b4f3dc4b19e7b
