Rename unittest_file item to unittest

- constants.py: TYPE_UNITTEST_FILE → TYPE_UNITTEST, cmd "unittest_file" → "unittest"
- All Python files updated: test_item_unittest.py, test_set.py, test_init.py,
  terminal.py, report_export_txt.py, test_tree_item.py
- All .tum files updated (examples, validation, doc)
- Sphinx doc: unittest_file_test_item.rst → unittest_test_item.rst,
  all references updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

doc/examples/dummy/dummy.tum

@@ -15,7 +15,7 @@ main:
                 - $(reference_1)
                 - $(reference_2)
             report_show_success: true
-        - unittest_file:
+        - unittest:
               name: Test 1
               test_file: dummy.py
               doc: |
@@ -23,7 +23,7 @@ main:
                   Voilà...
         - sleep:
             {name: Sleep between one and two, timeout: 10, dialog: true}
-        - unittest_file:
+        - unittest:
             {name: Test 2, test_file: dummy.py,execute_on_stop: true}
         - loop:
             name: Cycle Temperature