Refactor test/variable.jl into functions as opposed to nested testsets

[odow]

Extracted from #2277

[codecov]

Codecov Report

Merging #2307 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2307      +/-   ##
==========================================
- Coverage   91.37%   91.32%   -0.05%     
==========================================
  Files          42       42              
  Lines        4255     4255              
==========================================
- Hits         3888     3886       -2     
- Misses        367      369       +2     

Impacted Files	Coverage Δ
src/variables.jl	96.82% <0.00%> (-0.80%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update d9beefd...ba67a47. Read the comment docs.