Stop testing 'tests-require' functionality (#661)

This field was removed from setuptools v72.0.0. We'll continue to read
the field in colcon when the underlying setuptools version supports it,
but this change drops the testing for its functionality.

test/test_package_identification_python.py

@@ -74,7 +74,6 @@ def test_identify():
             'install_requires =\n'
             '  runA > 1.2.3\n'
             '  runB\n'
-            'tests_require = test == 2.0.0\n'
             'zip_safe = false\n'
             '[options.extras_require]\n'
             'test = test2 == 3.0.0\n'
@@ -93,7 +92,7 @@ def test_identify():
         assert desc.dependencies['run'] == {'runA', 'runB'}
         dep = next(x for x in desc.dependencies['run'] if x == 'runA')
         assert dep.metadata['version_gt'] == '1.2.3'
-        assert desc.dependencies['test'] == {'test', 'test2', 'test3', 'test4'}
+        assert desc.dependencies['test'] == {'test2', 'test3', 'test4'}
 
         assert callable(desc.metadata['get_python_setup_options'])
         options = desc.metadata['get_python_setup_options'](None)