Slightly relax tolerance in a test case of Apply1InteriorArbitrary

[mhucka]

The test case in third_party/qsim/tests/mps_simulator_test.cc on line 262 fails if the Eigen linear algebra library is updated to a recent version available from https://gitlab.com/libeigen/eigen. Specifically, commit 59498c96 in the Eigen repo (which can be seen at https://gitlab.com/libeigen/eigen/-/merge_requests/1663) changed the code in Eigen/src/Core/arch/Complex.h for pmul() to use a fused multiply-add-subtract (FMA) instruction (fmaddsub) instead of the previous separate multiply and add-subtract instructions. The new code improved accuracy for many of the tests in mps_simulator_test.cc, slightly worsened accuracy for some of the tests, and in the case of the one on line 262, made it cross the absolute tolerance value of 1e-5 by a very small amount. After considering various alternatives, the least-worst option seems to be to increase the tolerance for this one case.

Note: the qsim repository on GitHub has its own vendored copy of Eigen, so this change is not strictly necessary; however, if we ever update qsim's internal copy of Eigen, then this change will become relevant.

The original debugging & solution are the work of @pavoljuhas.

[pavoljuhas]

The CI errors show on the master branch too and are unrelated to this change. Perhaps this can be merged as is for the sake of consistency with downstream fix?

[mhucka]

That sounds reasonable to me.

It currently needs reviewer approval from @sergeisakov. (Sergei, this is the same tiny change as was made in the internal code base recently for the same problem, so hopefully will take next to no time to review.)