chore: Remove test files' direct execution code

This subtracts from coverage, and it's not much of a time saver:
`python tests/extensions/test_version.py` isn't much less typing than
`python -m unittest tests/extensions/test_version.py`.

Author: l0b0

tests/extensions/test_sar.py

@@ -203,7 +203,3 @@ def test_should_raise_exception_when_passing_invalid_extension_object(
             SarExtension.ext,
             object(),
         )
-
-
-if __name__ == "__main__":
-    unittest.main()

tests/extensions/test_scientific.py

@@ -456,7 +456,3 @@ def test_set_doi_summaries(self) -> None:
 
         assert new_dois is not None
         self.assertListEqual([PUB2_DOI], new_dois)
-
-
-if __name__ == "__main__":
-    unittest.main()

tests/extensions/test_version.py

@@ -473,7 +473,3 @@ def test_ext_add_to(self) -> None:
         _ = VersionExtension.ext(collection, add_if_missing=True)
 
         self.assertIn(VersionExtension.get_schema_uri(), collection.stac_extensions)
-
-
-if __name__ == "__main__":
-    unittest.main()