DE in test_risch.py (test_recognize_log_derivative)

The following PR corrects the differential extension of the test case in test_risch.py under test_recognize_log_derivative. It changes DE = DifferentialExtension(extension={'D': [Poly(1, x), Poly(1, t)]}) to DE = DifferentialExtension(extension={'D': [Poly(1, x)]}). Also adds the case where DE = DifferentialExtension(extension={'D': [Poly(1, x), Poly(t**2 + 1, t]})