Increase tolerance in n_qubit_tomography_test' (#2774)
Increase tolerance in n_qubit_tomography_test' (#2774)

I just got CI failure with this test failing with message 
```
assert np.linalg.norm(error_rho) < 0.05
AssertionError: assert 0.055035732500366044 < 0.05
```
quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq,quantumlib/Cirq,balopat/Cirq,quantumlib/Cirq,balopat/Cirq