Adaptation tests fail if unlucky with random seed

[torfjelde]

AdvancedHMC.jl/test/adaptation.jl

Line 148 in 564af93

@test res.adaptor.pc.var ≈ diag(Σ) rtol=0.2

fails sometimes, e.g. https://github.com/TuringLang/AdvancedHMC.jl/runs/3073138476.

To determine the fix/cause needs further investigation.