executor: fix tracking of active jobs

[mdonadoni]

Fix an issue where workflows would be stuck waiting for already-finished jobs. This happened because the list of active jobs was incorrectly overwritten instead of being extended, causing the executor to lose track of some of them.

Closes #59

[codecov-commenter]

Codecov Report

Merging #60 (7b57364) into master (ff43fba) will not change coverage. The diff coverage is 0.00%.

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Additional details and impacted files

[](https://app.codecov.io/gh/reanahub/reana-workflow-engine-snakemake/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub) ```diff @@ Coverage Diff @@ ## master #60 +/- ## ====================================== Coverage 3.84% 3.84% ====================================== Files 6 6 Lines 182 182 ====================================== Hits 7 7 Misses 175 175 ``` | [Impacted Files](https://app.codecov.io/gh/reanahub/reana-workflow-engine-snakemake/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub) | Coverage Δ | | |---|---|---| | [reana\_workflow\_engine\_snakemake/executor.py](https://app.codecov.io/gh/reanahub/reana-workflow-engine-snakemake/pull/60?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=reanahub#diff-cmVhbmFfd29ya2Zsb3dfZW5naW5lX3NuYWtlbWFrZS9leGVjdXRvci5weQ==) | `0.00% <0.00%> (ø)` | |