Impropve flaky unittest

[mrossinek]

The test_backpropagate_timeout can be quite flaky at times. Specifically, the check which asserts that the time spent on backpropagation is smaller than the specific maximum limit:

qiskit-addon-obp/test/test_backpropagation.py

Line 896 in 58ec753

assert (t2 - t1) < 2

We should consider ways of removing this flakiness in a way that is not affected by performance improvements of the underlying code (if that is possible).