Increase threshold for expected value in test_non_linear

[taketwo]

Fixes #2288.

[SergioRAgostinho]

Quick debate: I'm questioning removing that check entirely? It's not useful.

[taketwo]

I think we need to verify that the call to segment() returns at least some inliers.

[taketwo]

Though we may relax the threshold even more to avoid problems in future.

[SergioRAgostinho]

I assumed so and its a valid approach. Alternatively I was thinking if setting a fixed random seed before would also prevent problems.

[taketwo]

Fixing the seed is a good approach, but I don't think it's possible here. SampleConsensus class has a private member variable rng_ containing the random generator and we don't have access to it to set the seed.