Skip test when running `pytest --pdb`
Skip test when running `pytest --pdb`

Our `--pdb` hook for debugging greenlets interferes with the test logic
in `test_matrix_tx_error_handling`. To spare other developers from
debugging a non existing test failure, I marked the test to skip if run
with the `--pdb` option.

hackaugusto/raiden,hackaugusto/raiden