Fix example test

[ali-ramadhan]

Cherry picked some commits from PR #381 to fix the example test.

One thing that's going to be annoying about testing example scripts is that they evaluate in global scope when including them, so it's going to be easy to get variable name conflicts...

Not sure how to get around this as we agreed not to use functions for examples.

[codecov]

Codecov Report

Merging #424 into master will increase coverage by 3.56%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #424      +/-   ##
==========================================
+ Coverage   74.89%   78.46%   +3.56%     
==========================================
  Files          25       25              
  Lines        1414     1416       +2     
==========================================
+ Hits         1059     1111      +52     
+ Misses        355      305      -50

Impacted Files	Coverage Δ
src/poisson_solvers.jl	97.65% <0%> (-0.77%)	⬇️
src/boundary_conditions.jl	86.88% <0%> (+1.63%)	⬆️
src/utils.jl	85.57% <0%> (+12.76%)	⬆️
...rc/TurbulenceClosures/velocity_tracer_gradients.jl	86.23% <0%> (+16.51%)	⬆️
...ures/verstappen_anisotropic_minimum_dissipation.jl	100% <0%> (+38%)	⬆️

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[glwagner]

Might be able to wrap each example in a module within the temporary modified script that we use to run the test.

[glwagner]

Check out #425. Would be a good place to implement auto-moduling.