Add more class fields tests

[chicoxyzzy]

Currently tests don't cover computed fields
https://bugzilla.mozilla.org/show_bug.cgi?id=1535166

[afmenez]

Wasn't this fixed by #1440?

[chicoxyzzy]

Not all cases were covered by #1440
For example

class C {
    2 = 2;
    0b01 = 2;
    "hello" = 2;
}

and presence of property x in instance of

class C {
    x;
}

are still worth testing IMO

[chicoxyzzy]

@evseevdev would you be open to add more tests? If so, you may ping me in Telegram again if you need more information.