test: improve coverage and raise threshold to 80% (#154)

tests/test_api_exports.py

@@ -0,0 +1,7 @@
+from takopi import api
+
+
+def test_api_exports() -> None:
+    assert api.TAKOPI_PLUGIN_API_VERSION == 1
+    assert "TransportRuntime" in api.__all__
+    assert api.TransportRuntime is not None