tests: expand correctness coverage

[rgerhards]

Summary

• exercise serializer buffer boundaries, callback short writes, and FILE output
• add file/FD and array API regression coverage
• remove unsupported and orphaned test artifacts

Validation

• make check (20 tests)
• make distcheck
• coverage build: 80.7% production lines, 90.2% production functions

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Summary by cubic

Expands test coverage for JSON serialization and array/file APIs, including buffer boundaries, short writes, and FILE/FD I/O. Cleans up unsupported tests and updates the test suite.

• New Features
  • Added array API regression tests: add/get/sort/bsearch, put_idx/del_idx, and sparse slots.
  • Expanded serializer coverage: buffered size boundaries, short-write handling via callbacks, FILE* writes, pretty/plain output, and size queries.
  • Added json_util tests for to/from file and from FD with offset; removed obsolete NUL-string and custom serializer tests and updated tests/Makefile.am.

^{Written for commit 3635695. Summary will update on new commits.}