As illustrated in #14230 tests can contain unreachable code which then produces false positives.

Looking at codecov results for test files is probably a good start. For instance, there may be issue in,

• test_countvectorizer_custom_vocabulary_repeated_indices, test_countvectorizer_custom_vocabulary_gap_index

(I have checked only a few test files).

cc @VarIr in case you are interested in looking into this (possibly in another PR)..