zmq4: extend ZMTP greeting tests for various version cases

[sbinet]

Updates #56.

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by 0.02%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   54.94%   54.92%   -0.03%     
==========================================
  Files          26       26              
  Lines        1931     1941      +10     
==========================================
+ Hits         1061     1066       +5     
- Misses        778      782       +4     
- Partials       92       93       +1

Impacted Files	Coverage Δ
protocol.go	77.23% <81.81%> (+0.24%)	⬆️
conn.go	54.73% <0%> (-1.06%)	⬇️

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