Adding to unity substitution set

[victorli2002]

Filling out unity_substitution_set with tests

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This change is 

[victorli2002]

Reviewable status: 0 of 437 files reviewed, 1 unresolved discussion

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lib/substitutions/test/src/substitutions/unity_substitution_set.cc line 34 at r1 (raw file):

  TEST_CASE("create_replicate_linear_combine") {
    SUBCASE("use_bias = false") {

Having trouble getting this test to work, not sure where needs to be fixed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (fd3d7f1) to head (8301bed).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

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##   master   #1594   +/-   ##
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