test: write test output to tmp_path, untrack generated FITS artifacts

[Jammy2211]

Summary

test_autoarray wrote test output into tracked paths, so any change to the autonerves
FITS writer rewrote a committed binary and dirtied the tree for every contributor. #483
demonstrated it: the PyAutoNerves#155 header-comment fix ([""] -> "") silently modified
structures/arrays/files/array/output_test/array.fits the moment the autonerves floor moved —
identical cards, values, data and byte size, but a modified tracked file that had to be either
committed into an unrelated PR or left dirtying the tree.

This applies the durable fix uniformly rather than patching file-by-file again. The nine tests
that wrote into an output_test/ directory now take pytest's tmp_path and drop their
rmtree/makedirs preamble, and the 13 exposed artifacts are untracked. After a full run no
output_test/ directory is created anywhere in the source tree.

The scope narrowed after auditing the inventory

Of the 13 tracked files, only one was a live output — the array.fits that #483 flipped. The
other 12 were orphans: no test referenced their paths, and a repo-wide grep for their basenames
returned nothing. They were residue from tests deleted long ago — as were the two file-by-file
.gitignore lines, which named generated files for a test that no longer exists.

The issue's open question about test_uniform_1d.py:12 resolves the same way: that path was
written, not read. No file in the set was a real input, so nothing needed relocating instead of
untracking.

Two incidental fixes fall out

• test_uniform_1d and test_uniform_2d both rmtree'd the same shared output directory — an
  ordering hazard between two test modules that tmp_path removes.
• The three imaging tests taking the test_data_path fixture wrote their generated FITS into the
  source tree through it; they now use tmp_path and the fixture is gone.

API Changes

None — this PR touches test_autoarray/ and .gitignore only. No module under autoarray/ is
modified, so there is no public-API surface change and no downstream workspace migration.

Test Plan

• Full suite from a clean checkout: 1177 passed, 55 skipped — identical to the pre-change
  baseline on main (1177/55).
• Suite run twice in a row with git status --porcelain asserted unchanged after each. The
  second run is what would catch a test that had silently depended on a committed artifact: it
  would fail on run 1, not run 2.
• find test_autoarray -path "*output_test*" returns nothing after a full run — the tests no
  longer write into the source tree at all.
• No tracked file remains under any output_test/ path (git ls-files | grep output_test).

Sibling repos

Checked as the issue asked, and not widened into this PR. Neither PyAutoGalaxy nor PyAutoLens
uses the output_test/ name, and both track zero test-output artifacts — this defect is
autoarray-only. PyAutoLens does have a smaller, different problem (its tests write to
test_autolens/{imaging,interferometer}/data_temp/, which its .gitignore does not cover — only
the integration/ copies), filed separately rather than mixed in here.

Full API Changes (for automation & release notes)

Removed

• test_autoarray/dataset/imaging/test_dataset.py::make_test_data_path — the test_data_path
  pytest fixture, which created files/array/output_test/ in the source tree. Its three consumers
  now take tmp_path. Test-suite-internal; not part of the autoarray package.

Added

• Nothing.

Migration

• None. No autoarray public symbol is added, removed or changed.

Closes #494

Generated by the PyAutoLabs agent workflow.

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