Fix mctest TypeError on re-runs: displaytime never restored from JSON

[mads-bertelsen-agentic]

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Please describe what your PR is adding in terms of features or bugfixes:

mctest aborts a whole test run with

TypeError: %d format: a real number is required, not NoneType

in the "Display OK/FAILED" status line whenever it re-enters an instrument test dir that already contains results from an earlier run. Reported by users on PR #2622 (crash at the "Display FAILED" line).

How it happens:

• If the per-instrument test dir already exists - e.g. a re-run with the same --testdir and --uid, which is exactly how the CI workflows invoke mctest - shutil.copytree fails and mctest falls back to InstrExampleTest.load() to rebuild the test objects from the JSON files saved by the previous run.
• load() restored every stored field except "displaytime", leaving the test object with displayed= but displaytime=None. The "binary already exists" branch of the compile loop skips the mcdisplay step as well, so displaytime is never (re)measured.
• The run/status loop then formats test.displaytime with %d, which raises TypeError and kills the run before any results are written. The "Display FAILED" variant (the one seen on PR Powder n and single crystal minorfixes #2622) occurs when the first run recorded displayed=false, e.g. because mcdisplay hit the 60 s --displaymax timeout - seen for MCPL instruments such as ESS_BEER_MCPL / ESS_KVASIR / ESS_butterfly_MCPL_test.

Recreate the error (before this fix):
mctest --testdir tmp --instr ESS_KVASIR -n 1e4 --uid reproA
# 1st run: SUCCESS, saves results JSON
mctest --testdir tmp --instr ESS_KVASIR -n 1e4 --uid reproA
# 2nd run: "WARNING: Skipped ESS_KVASIR test - did ... exist
# already??" followed by the TypeError traceback above
The FAILED-branch variant is forced by making the display time out in the first run, e.g. adding --displaymax 0.

Fix:

• InstrExampleTest.load() now restores displaytime from the saved JSON (via obj.get, so JSON files written without the key still load).
• The Display OK/FAILED status lines fall back to 0 s when displaytime is None, which also covers a binary present without saved display time (e.g. a first run interrupted between compile and saving).

Verified: both re-run scenarios above now complete with SUCCESS, and fresh mctest runs are unaffected.

Note: get('displaytime') used as it won't fail if the field is absent on an old test.

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Development OS / boundary conditions

Please describe what OS you developed and tested your additions on, and if any special dependencies are required:
OS X 14.6.1 (23G93)

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PR Checklist for contributing to McStas/McXtrace

• My work touches the code-generator in mccode/src

  • I have added reasoning and documentation for the change through an ADR record in our GRAMMAR section
  • I am attaching test output in the comments

• My work touches / adds to the runtime lib code (.c,.h etc in multiple locations

  • I am have added reasoning and documentation for the change below
  • I am attaching test output in the comments

• My PR is meant to fix a specific, existing issue

  • I have indicated the issue number here: Mentioned in PR Powder n and single crystal minorfixes #2622
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[willend]

Argh, dumb detail. Thanks for the PR, let's just get it merged....