Fix/mock payload generator plural double nest

[vchernovatl]

Summary

MockPayloadGenerator produced a double-nested list ([[item]] instead of
[item]) when the same plural linked field is selected twice at one path —
once unconditionally and once in a sibling @include/@skip condition or
abstract inline fragment. The normalizer then couldn't read the leaf fields
(Payload did not contain a value for field ...).

Cause: in _mockLink's generateDataForField, the whole prior array was
passed as each element's previous value, so it got wrapped again. Fix: index
the prior array by index for plural fields (as fieldPath already is).

Fixes #5258.

Test plan

Added regression tests for the @include and concrete+abstract-fragment shapes
(via testGeneratedData, so the normalizer runs under disallowWarnings()).

• yarn jest — 243 suites / 3739 tests pass (fail without the fix)
• flow, prettier --check, eslint — clean

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