Improve test coverage of tasks.jl

[giordano]

With the goal of improving test coverage of the whole library, I'm starting from the file I've already worked on, tasks.jl. I also started using @test_logs, to actually test the content of the log messages, making the tests visually less noisy and in the end more robust.

Thanks to test coverage I also realised that there were a couple of coerce methods that were not actually being called because of wrong signature: the default less efficient methods were being called instead, but the result was correct in any case.

The only line that is not tested is the very last one, the unsupervised function, I'm not sure what could be a good test for it.

[codecov-io]

Codecov Report

Merging #135 into master will increase coverage by 0.92%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage    69.1%   70.02%   +0.92%     
==========================================
  Files          18       18              
  Lines        1191     1191              
==========================================
+ Hits          823      834      +11     
+ Misses        368      357      -11

Impacted Files	Coverage Δ
src/tasks.jl	96.61% <100%> (+18.64%)	⬆️

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[ablaom]

Thanks for that! Looks good to me.