Clean-up release 1.4 branch

[oxinabox]

Before I try and backport anything,
I want 1.4 to be a bit cleaner.

This PR is best reviewed 1 commit at a time

• first commit removes all the if VERSION, and moves the tests about to match others
• second commit moves all the code around to match structure of other branchs (short make backporting just work, i hope)

Turns out 1.4->1.6 actually can be done without any if VERSION

@mmiller-max or perhaps @nickrobinson251 can I trouble for a review?
I would like to have this done so i can backport #20 and thus close the rest of #19

[codecov-commenter]

Codecov Report

Merging #22 (236d37a) into release-1.4 (53390df) will increase coverage by 19.45%.
The diff coverage is 79.51%.

@@               Coverage Diff                @@
##           release-1.4      #22       +/-   ##
================================================
+ Coverage        60.30%   79.76%   +19.45%     
================================================
  Files                6        4        -2     
  Lines              131       84       -47     
================================================
- Hits                79       67       -12     
+ Misses              52       17       -35     

Impacted Files	Coverage Δ
src/TestEnv.jl	100.00% <ø> (+87.50%)	⬆️
src/common.jl	75.00% <75.00%> (ø)
src/activate_set.jl	80.00% <80.00%> (ø)
src/activate_do.jl	100.00% <100.00%> (ø)

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[mmiller-max]

Looks good to me!