Switch to testsets

[timholy]

This may make the tests more reproducible, since we set the seed inside a @testset. See https://github.com/JuliaStats/NMF.jl/pull/70#issuecomment-1396090714

Note that the random number generator is not guaranteed to give the same sequence across all Julia versions, so version-specific failures are still possible, but at least they should be reproducible.

It will also fix a (repeated) warning:

┌ Warning: Assignment to `X` in soft scope is ambiguous because a global variable by the same name exists: `X` will be treated as a new local. Disambiguate by using `local X` to suppress this warning or `global X` to assign to the existing global variable.
└ @ ~/.julia/dev/NMF/test/alspgrad.jl:15

[codecov-commenter]

Codecov Report

Base: 92.38% // Head: 92.38% // No change to project coverage :thumbsup:

Coverage data is based on head (9ef3fec) compared to base (0cc9173). Patch has no changes to coverable lines.

Additional details and impacted files

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