RFC: EffectRouterSpec

[jeppes]

I started implementing a given-when-then style test utility for the EffectRouter with the following syntax:

EffectRouterSpec.given(effectRouter)
    .when(.effect1)
    .then(
        expectEvents(.event1, .event2),
        expectSideEffects {
            // Verify that side effects were performed
            ... 
        }
    )

This is not yet documented. I am sharing this PR mainly to get feedback on the API.

@JensAyton @pettermahlen

[codecov]

Codecov Report

Merging #156 into master will decrease coverage by 0.19%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   94.26%   94.07%   -0.20%     
==========================================
  Files          45       46       +1     
  Lines        1256     1283      +27     
==========================================
+ Hits         1184     1207      +23     
- Misses         72       76       +4     

Flag	Coverage Δ
#ios	94.07% <85.18%> (-0.20%)	⬇️
#macspm	94.02% <85.18%> (-0.20%)	⬇️

Impacted Files	Coverage Δ
MobiusTest/Source/EffectRouterSpec.swift	85.18% <85.18%> (ø)

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