Add empty extras test to modules without needs

Since version 4.36.0 tox is validating extras option.

modules/timesheet_cost/setup.py

@@ -47,6 +47,8 @@ for dep in info.get('depends', []):
         requires.append(get_require_version('trytond_%s' % dep))
 requires.append(get_require_version('trytond'))
 
+tests_require = []
+
 setup(name=name,
     version=version,
     description='Tryton module to add cost on timesheet',
@@ -115,6 +117,9 @@ setup(name=name,
     license='GPL-3',
     python_requires='>=3.9',
     install_requires=requires,
+    extras_require={
+        'test': tests_require,
+        },
     zip_safe=False,
     entry_points="""
     [trytond.modules]