Our use case is T190829, for which we want to add this configuration to LocalSettings.php:

$wgEnableUploads = true;

Ideally, we can install this custom config and launch a corresponding PHP server for just this one extension's browser tests. I can imagine that we use a LocalSettings.d similar to how mediawiki-vagrant is configured, or maybe we can just supply the configuration on the command-line to reduce potential race conditions if servers are launched for other extensions.

This just became possible with T199116.

Precedents

The Ruby browser test running script would cat directly onto LocalSettings.php,
https://phabricator.wikimedia.org/source/integration-config/browse/master/dockerfiles/quibble-stretch-bundle/mwselenium.sh$35-39

Unfortunately, this can't be cleaned up easily and will break isolation between tests.