[Toybox] avoiding testing shell builtins

[Rob Landley]

On 6/27/19 1:10 PM, enh wrote:
> this also fixed the echo test failure. now at 68 passed, 11 failed
> (though that includes the split test failure that's really just "mksh
> doesn't support that bash extension").

I'm actively working on a shell that _should_ support the bash extension (which
is why I used it in the tests), but that's got another month to go before it's
usable and you're not using it this year even if it's perfect, so...

Sigh. Applied.

Rob