[feature/removeVariable] replace `Variable` on `BehaviourRelay`

[M0rtyMerr]

Hi, thank you for your great work! This PR allows migrate ReactorKit on Behavior & Publish Relays instead of own types.

[codecov-io]

Codecov Report

Merging #16 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #16   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           4      4           
  Lines          78     78           
=====================================
  Hits           78     78

Impacted Files	Coverage Δ
Sources/RxExpect/RxExpect.swift	100% <100%> (ø)	:arrow_up:

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