fix: testmodule: find the scenario file under each name twister reads

[tobiaskaestner]

Summary

The scenario table of .. testmodule:: read only <module>/testcase.yaml. Newer Zephyr trees name this file tests.yaml. So on such a tree, every test module logged "could not read …testcase.yaml", and the table was missing.

This PR takes the first of testcase.yaml, tests.yaml and sample.yaml that exists. These are the names that twister reads, in twister's order (scripts/pylib/twister/twisterlib/testplan.py). Projects on an older Zephyr keep working.

• rst_builders.find_scenario_yaml() finds the file. When no file exists, the warning names each file it tried.
• .. testmodule:: records all three names as inputs. A scenario file that is added or renamed later also re-reads the document.
• The two manual pages that named testcase.yaml now name all three.

Test

• Unit suite: 403 passed (399 before). The new tests cover these cases:
  • only tests.yaml exists;
  • only sample.yaml exists;
  • all three exist, and testcase.yaml wins;
  • no file exists, and the warning names the files.
• Clean build of the Zephyr safety document set (Zephyr with tests.yaml), main compared with this branch:
  • "could not read …testcase.yaml": 435 → 0;
  • test specification pages with a scenario table: 0 → 188;
  • Sphinx warnings, doc-check, testspec-check and satisfies-check: no change;
  • the test specification needs keep the same 1207 ids. Only lineno changes, because of the new tables.

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