SigTerms cannot run on fields that are not searchable, and SigText
cannot run on fields that do not have analyzers. Both of these
situations fail today with an esoteric exception, so this just formalizes
the constraint by throwing an IllegalArgumentException up front.
In practice, the only affected field seems to be the `binary` field,
which is neither searchable or has a default analyzer (e.g. even numeric
and keyword fields have a default analyzer despite not being tokenized)
Adds supported-type tests, and makes some changes to the test itself
to allow testing sigtext (indexing _source).
Also a few tweaks to the test to avoid bad randomization (negative
numbers, etc).
79ac69cfa3