zmq4: add r/w test for greeting

[sbinet]

Updates #56.

[codecov-io]

Codecov Report

Merging #60 into master will increase coverage by 0.57%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   54.37%   54.94%   +0.57%     
==========================================
  Files          26       26              
  Lines        1933     1931       -2     
==========================================
+ Hits         1051     1061      +10     
+ Misses        784      778       -6     
+ Partials       98       92       -6

Impacted Files	Coverage Δ
protocol.go	76.99% <100%> (+10.03%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 78ce94b...411132b. Read the comment docs.