`rustdoc-ui` tests fail when `x.py` is run with `--pass check`

[petrochenkov]

E.g. ./x.py test --bless --stage 1 --pass check src/test/rustdoc-ui.

Errors look like this:

stdout:
------------------------------------------

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stderr:
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error: Unrecognized option: 'emit'


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It prevents running rustdoc-ui tests together with other tests suites for which --pass check makes sense.
Usually test suites ignore this option if they cannot work with it.