Can't set a default theme in a FunctionalJavascript test with $defaultTheme class property
I wound up installing a barebones install with drush si and exporting certain configuration to an install profile. After adding this to drush si in CI and local, all functional javascript browser tests work without complaints. 

I double checked and the parent BrowserTestBase class does have defaultTheme and profile properties, so I don't know why those were not installed.