Wine uses gecko 2.47.2 since wine commit
70567d9f2d32fa3f052609051e9913344f24c42a, which
is wine versions 6.0 up through current (6.9)
Add a test verifying that the "Can't find Gecko" error message does
not appear. A positive test of HTML rendering would be better (eg:
would be robust against changes in the error message string), but this
test is easy to set up & much better than nothing.
