Add DSP tests

[jishnub]

Since conv methods may be excised from InfiniteLinearAlgebra, it's best to have tests for these here. The specialized conv methods for AbstractFill are unnecessary, and are actually less efficient than the fallback ones.

[codecov]

Codecov Report

Merging #289 (88eddfa) into master (fef31cd) will decrease coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is n/a.

❗ Current head 88eddfa differs from pull request most recent head 3f8ca5c. Consider uploading reports for the commit 3f8ca5c to get more accurate results

@@            Coverage Diff             @@
##           master     #289      +/-   ##
==========================================
- Coverage   99.88%   99.88%   -0.01%     
==========================================
  Files           7        5       -2     
  Lines         871      869       -2     
==========================================
- Hits          870      868       -2     
  Misses          1        1              

see 4 files with indirect coverage changes

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