Add `atol` to addmul tests (#56210)
This avoids the issues as in
https://github.com/JuliaLang/julia/issues/55781 and
https://github.com/JuliaLang/julia/issues/55779 where we compare small
numbers using a relative tolerance. Also, in this PR, I have added an
extra test, so now we compare both `A * B * alpha + C * beta` and `A * B
* alpha - C * beta` with the corresponding in-place versions. The idea
is that if the terms `A * B * alpha` and ` C * beta` have similar
magnitudes, at least one of the two expressions will usually result in a
large enough number that may be compared using a relative tolerance.
I am unsure if the `atol` chosen here is optimal, as I have ballparked
it to use the maximum `eps` by looking at all the `eltype`s involved.
Fixes #55781
Fixes #55779
