LinearOperatorTestCase's _test_triangular_linear_op_inv_quad_logdet is skipped and fails otherwise

[SebastianAment]

LinearOperatorTestCase's _test_triangular_linear_op_inv_quad_logdet is not executed in the current test suite because of the leading underscore. When this underscore is removed, the test executes but fails.

In particular, the test calls _test_inv_quad_logdet and checks if this uses a call to cholesky. However, derived test classes like TestLowRankRootAddedDiagLinearOperator, TestLowRankRootAddedDiagLinearOperatorBatch and TestSumKroneckerLinearOperator apparently don't use Cholesky-based inference for this step (see here for the low rank test), which makes the test fail.