It appears that DataAccessSnakFormatterOutputFormatTest can fail in certain circumstances.
When certain code is run before this test is, the tests suddenly start to fail.

Apparently, even though a new 'reset' client (WikibaseClient::getDefaultInstance( 'reset' )) is constructed, it doesn't appear that the client is completely reset & some code still sticks around.
The tests can be made to fail by adding this to setUp:

WikibaseClient::getDefaultInstance()->getDataAccessSnakFormatterFactory()->newWikitextSnakFormatter( Language::factory( 'en' ), new HashUsageAccumulator() );

More details can be found on this patchset, in Lucas' notes on PS9: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/WikibaseMediaInfo/+/556981/9