Validating stages option

[luizbafilho]

this PR validates the stages option, making sure only valid stages are being passed down to the test.

fixes #547

[na--]

LGTM

[codecov-io]

Codecov Report

Merging #615 into master will decrease coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #615      +/-   ##
==========================================
- Coverage   62.23%   62.07%   -0.17%     
==========================================
  Files          97       97              
  Lines        7510     7512       +2     
==========================================
- Hits         4674     4663      -11     
- Misses       2577     2589      +12     
- Partials      259      260       +1

Impacted Files	Coverage Δ
lib/options.go	95.16% <100%> (+0.07%)	⬆️
stats/units.go	50% <0%> (-50%)	⬇️
stats/stats.go	41.11% <0%> (-6.12%)	⬇️

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