MapperTestCase has a check that if a field mapper supports stored fields,
those stored fields are available to index time scripts. Many of our mappers
do not support stored fields, and we try and catch this with an assumeFalse
so that those mappers do not run this test. However, this test is fragile - it
does not work for mappers created with an index version below 8.0, and it
misses mappers that always store their values, e.g. match_only_text.
This commit adds a new supportsStoredField method to MapperTestCase,
and overrides it for those mappers that do not support storing values. It
also adds a minimalStoredMapping method that defaults to the minimal
mapping plus a store parameter, which is overridden by match_only_text
because storing is not configurable and always available on this mapper.
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