Drop ReTest test dep

[jakobnissen]

The reason to drop this is:

• To make it more in line with all other BioJulia packages
• To simplify the test file layout. Even though this commit now doesn't greatly simplifies it, we can do this in the future
• To not rely on ReTest being maintained
• To not have two separate projects to store test dependencies. Before this commit, the deps were stored both in ./Project.toml and test/Project.toml.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (45854a4) 89.72% compared to head (83ee4f9) 89.72%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #112 +/- ## ======================================= Coverage 89.72% 89.72% ======================================= Files 15 15 Lines 691 691 ======================================= Hits 620 620 Misses 71 71 ``` | [Flag](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/112/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/112/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | `89.72% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#carryforward-flags-in-the-pull-request-comment) to find out more. | [Files Changed](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/112?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia) | Coverage Δ | | |---|---|---| | [src/workload.jl](https://app.codecov.io/gh/BioJulia/FASTX.jl/pull/112?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=BioJulia#diff-c3JjL3dvcmtsb2FkLmps) | `100.00% <100.00%> (ø)` | |

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