Rules no-global-variable, no-suite-variable, and no-test-variable

[Lakitna]

Closes #1117

I'm not 100% sure about the tone of voice for the docs 🤔

[codecov]

Codecov Report

Attention: Patch coverage is 97.29730% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.92%. Comparing base (f62508c) to head (4fdbdf8).
Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
robocop/checkers/misc.py	97.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1137      +/-   ##
==========================================
- Coverage   97.02%   96.92%   -0.10%     
==========================================
  Files          37       39       +2     
  Lines        4670     5014     +344     
==========================================
+ Hits         4531     4860     +329     
- Misses        139      154      +15     

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[bhirsz]

Thanks for the PR, look good enough! The tone in the docs looks fine, though I'm not best doc writer .. :) If needed we can always improve it over time. I have left some comments but it's mostly nitpicks or improvements ideas (some stuff I've noticed I could also do with my rules, it's easier to see it if I look into someone else code).

[Lakitna]

Those where some good suggestions :)

I think I covered them all