Some tests will not work if certain aspects of MediaWiki are changed by extensions, e.g. requiring captchas for edits or account creation. Such extensions need to set a flag that is exposed by SiteInfo, so the respective tests can be skipped in the API test suite.
In addition, the API test suite should not run at all if SiteInfo does not expose a "test mode" flag. That flag should be enabled by a MediaWiki configuration setting. If that flag is not present, the test suite should abort presenting a message that instructs the user to include DevelopmentSettings.php in their LocalSettings.php.