Fixed potential bug where RNG isn't instantiated

[neworderofjamie]

If custom update variables were the only place in a model where an RNG was required for initialization, this wouldn't be detected and the model wouldn't compile. Have added extra unit tests and beefed up existing ones to test further up.

[codecov]

Codecov Report

Base: 89.38% // Head: 89.34% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (e6d85fb) compared to base (5b14794).
Patch coverage: 55.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #540      +/-   ##
==========================================
- Coverage   89.38%   89.34%   -0.04%     
==========================================
  Files          73       73              
  Lines       10747    10767      +20     
==========================================
+ Hits         9606     9620      +14     
- Misses       1141     1147       +6     

Impacted Files	Coverage Δ
src/genn/genn/code_generator/backendSIMT.cc	89.21% <50.00%> (-0.47%)	⬇️
src/genn/backends/single_threaded_cpu/backend.cc	90.80% <60.00%> (-0.35%)	⬇️
src/genn/genn/customUpdate.cc	93.54% <0.00%> (+2.41%)	⬆️

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[tnowotny]

Looks sensible - nice catch.
Somehow I am having a deja-vu of having reviewed this one before but apparently not.