feat(reana-dev): change run-example to use gherkin-parser module (#817)

[ajclyall]

Run-example now uses the test files in the specification file to check the correctness of the results (by running the test command).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 29.80%. Comparing base (cd94d12) to head (add18d5). Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
reana/reana_dev/run.py	75.00%	2 Missing :warning:

Additional details and impacted files

[](https://app.codecov.io/gh/reanahub/reana/pull/817?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub) ```diff @@ Coverage Diff @@ ## master #817 +/- ## =========================================== + Coverage 19.22% 29.80% +10.57% =========================================== Files 26 26 Lines 2512 2483 -29 =========================================== + Hits 483 740 +257 + Misses 2029 1743 -286 ``` | [Files with missing lines](https://app.codecov.io/gh/reanahub/reana/pull/817?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub) | Coverage Δ | | |---|---|---| | [reana/reana\_dev/run.py](https://app.codecov.io/gh/reanahub/reana/pull/817?src=pr&el=tree&filepath=reana%2Freana_dev%2Frun.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub#diff-cmVhbmEvcmVhbmFfZGV2L3J1bi5weQ==) | `54.14% <75.00%> (+27.22%)` | :arrow_up: | ... and [9 files with indirect coverage changes](https://app.codecov.io/gh/reanahub/reana/pull/817/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub)

[mdonadoni]

PS: this PR needs to be rebased