Add unit tests for geopoint exports

[khalilurrrahmanridoykhan]

What changed

• adds export coverage for geopoint, start-geopoint, and background-geopoint
• verifies the conventional _latitude, _longitude, _altitude, and _precision columns
• covers complete four-component values, two-dimensional values, and missing values

Why

These three field types are mapped to FormGPSField, but their component-column export behavior was not directly covered together. The tests document and protect the expected export contract for downstream survey-data consumers.

Validation

• python -m pytest -q tests/test_exports.py — 72 passed, 1 deselected, 3 subtests passed
• git diff --check

Closes #328