Fix test.json schema for 'matrix'
Fix test.json schema for 'matrix'

I'm not sure what the current schema is describing, but it's not doing
anything useful to validate 'matrix' entries, as I discover when I come
to add something to that.

'matrix' is a dict with possible keys 'options' and 'exclude'.
'options' is a dict with arbitrary keys, whose's values are arrays

Once we describe those dicts correctly, also fix that 'val' keys may be
string, boolean or null, and the corresponding 'exclude' keys may be
string or boolean.

v2:
Also allow 'val' and 'exclude' to be an array of strings.

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