chore: organise tests according to guidelines

[faustbrian]

Follow https://github.com/ArkEcosystem/desktop-wallet/blob/3.0-react/CODING_STANDARDS.md to flatten the structure, make it more obvious what tests what without having to go through multiple nested directories and force smaller directories with focused components and tests.

[codecov]

Codecov Report

Merging #2146 into 3.0-react will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           3.0-react     #2146    +/-   ##
============================================
  Coverage     100.00%   100.00%            
============================================
  Files             45        97    +52     
  Lines            298       741   +443     
  Branches          49       113    +64     
============================================
+ Hits             298       741   +443     

Flag	Coverage Δ
#unit	100.00% <100.00%> (?)

Impacted Files	Coverage Δ
src/app/components/Button/Button.tsx	100.00% <ø> (ø)
src/app/components/Card/Card.tsx	100.00% <ø> (ø)
src/app/components/Card/CardControl.tsx	100.00% <ø> (ø)
src/app/components/Circle/Circle.tsx	100.00% <ø> (ø)
src/app/components/Divider/Divider.styles.ts	100.00% <ø> (ø)
src/app/components/Divider/Divider.tsx	100.00% <ø> (ø)
src/app/components/Form/FormLabel.tsx	100.00% <ø> (ø)
src/app/components/Select/Select.tsx	100.00% <ø> (ø)
...app/components/SideBar/SideBarItem/SideBarItem.tsx	100.00% <ø> (ø)
src/app/components/StepIndicator/StepIndicator.tsx	100.00% <ø> (ø)
... and 101 more

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