testing/thunderbird: fix FTBFS

error: casts cannot be followed by a field access
   --> servo/components/style/stylesheets/viewport_rule.rs:314:39
    |
314 |                     declaration!($min(value: shorthand.0, important: important)),
    |                                       ^^^^^^^^^^^^^^^^
...
323 |             "width" => ok!(shorthand -> [MinWidth, MaxWidth]),
    |                        -------------------------------------- in this macro invocation
    |
    = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
help: try surrounding the expression in parentheses
    |
314 |                     declaration!($min((value: shorthand).0, important: important)),