update theme-a11y-test to use login shorthand

[bph]

Replace
{
"step": "login"
},

with
"login":true,

And update the theme to twentytwentyfive and use the new themeData option, to read:

{
"step": "installTheme",
"themeData": {
"resource": "wordpress.org/themes",
"slug": "twentytwentyfive"
},
"options": {
"activate": true
}
}