test: add tests for escape-hex-write-handler

[nkuprins]

Related: #1000

Purpose of the pull request

EscapeHexCellWriteHandler has no tests. It used to have them, but they were deleted by accident (read #1000).

This PR is not a blind restore of those lost tests! I took a different approach; for example, @ParameterizedTest were used for better scalability and readability, etc.

What's changed?

• Two parameterised tables covering the escaping rule itself. Seven inputs that must be rewritten (_xHHHH_ to _x005F_xHHHH_) and eight that must be left exactly as typed.
• Non-idempotency: an already-escaped literal is escaped again.
• Guard tests for the inputs the handler must not act on: non-STRING cell data, a null WriteCellData, and a STRING holding a null value.
• One end-to-end case, parameterised over XLSX/XLS/CSV with @ExcelFormatSource: write a file through Fesod with the handler registered, read it back, and require the caller's literal. It sits in its own class tagged ROUND_TRIP, which keeps the rest tagged UNIT.

Checklist

• I have read the Contributor Guide.
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.