GPR/VFE equivalence checks fail on nightly

[simsurace]

See https://github.com/JuliaGaussianProcesses/ApproximateGPs.jl/runs/7748452163?check_suite_focus=true

Test Summary:             | Pass  Fail  Error  Broken  Total     Time
ApproximateGPs            |  181     1      1       2    185  2m25.0s
  sparse_variational      |  112     1      1            114  1m22.1s
    AbstractGPs interface |  104                         104    23.2s
    elbo                  |    4                           4     1.7s
    GPR/VFE equivalences  |    4     1      1              6    57.2s
      exact posterior     |    4     1                     5     2.7s
      optimised posterior |                 1              1    54.6s
  laplace                 |   69                    2     71  1m02.2s

[simsurace]

More importantly, they also fail on 1.8.0-rc3