Add in-place tests for merge_csr_mv! function

[johnzl-777]

merge_csr_mv! has the ability to add the result to the output vector that is passed in (versus just assigning values to it) but this wasn't explicitly tested by the merge_csr_mv! specific unit tests.

Unit tests now use rand instead of zeros to generate the y/input vector.

[codecov]

Codecov Report

Merging #17 (b3bf48d) into main (f382312) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #17   +/-   ##
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  Coverage   98.55%   98.55%           
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  Files           1        1           
  Lines          69       69           
=======================================
  Hits           68       68           
  Misses          1        1           

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