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/example_cycle.tum

@@ -14,7 +14,7 @@ main:
         key: report-key-2
         stop_on_failure: True
         steps:
-          - unittest_file:
+          - unittest:
               name: unittest item
               doc: |
                 The purpose of this unittest test item is to demonstrate
@@ -41,7 +41,7 @@ main:
               param:
                   - 123
 
-          - unittest_file:
+          - unittest:
               name: Unittest item
               test_file: dummy/dummy.py
               test_method:
@@ -98,7 +98,7 @@ main:
         name: Infine loop unittest step crashes
         stop_on_failure: True
         steps:
-          - unittest_file:
+          - unittest:
               name: Unittest item
               test_file: dummy/dummy.py
               test_method:
@@ -243,7 +243,7 @@ main:
         name: Infinite loop
         skipped: True
         steps:
-          - unittest_file:
+          - unittest:
               name: Unittest item
               test_file: dummy/dummy.py
               test_method: test_01_pass