Flexbox is still very much in progress, but things are working well
enough that we can enable it by default. It improves most pages that use
flexbox now.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.
