Add level and throw options to MLJTest.test

[ablaom]

Specifying level replaces load_only option, which makes the change breaking.

[codecov-commenter]

Codecov Report

Merging #7 (6073494) into dev (8588b36) will increase coverage by 0.51%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev       #7      +/-   ##
==========================================
+ Coverage   95.07%   95.58%   +0.51%     
==========================================
  Files           3        3              
  Lines         203      204       +1     
==========================================
+ Hits          193      195       +2     
+ Misses         10        9       -1     

Impacted Files	Coverage Δ
src/attemptors.jl	93.97% <100.00%> (+1.20%)	⬆️
src/test.jl	96.96% <100.00%> (+0.04%)	⬆️

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