Warning when changing the matrix multiplication mode

[schillic]

Since #728 there is a warning when changing the matrix multiplication mode (see for instance this recent CI run):

WARNING: Method definition default_matmul() in module IntervalArithmetic at /home/runner/work/IntervalArithmetic.jl/IntervalArithmetic.jl/src/IntervalArithmetic.jl:131 overwritten on the same line (check for duplicate calls to `include`).

[OlivierHnt]

Indeed, that was there before (actually with a lot more warnings). When calling configure yourself, it does not show.
[EDIT: This is a result of using @eval, but my understanding is that our usage is ok here]

[schillic]

So why does this function exist instead of just calling configure?

[OlivierHnt]

I should clarify: configure calls @eval default_matmul() = ...
However, the warning is only triggered when testing.