Update PythonRuntimeTests.swift

Author: philipturner

Tests/PythonKitTests/PythonRuntimeTests.swift

@@ -326,4 +326,16 @@ class PythonRuntimeTests: XCTestCase {
         }
         XCTAssertEqual(bytes, otherBytes)
     }
+    
+    func testPythonFunction() {
+        let pythonAdd = PythonFunction { (params: [PythonObject]) in
+            let lhs = params[0]
+            let rhs = params[1]
+            return lhs + rhs
+        }.pythonObject
+        
+        let pythonSum = pythonAdd(2, 3)
+        XCTAssertNotNil(Double(pythonSum))
+        XCTAssertEqual(pythonSum, 5)
+    }
 }