Ignore tests in codecov

[ascillitoe]

Update codecov settings to ignore tests.

Alibi equivalent of SeldonIO/alibi-detect#614.

[codecov]

Codecov Report

Merging #751 (0f6af93) into master (c3a8da2) will decrease coverage by 6.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #751      +/-   ##
==========================================
- Coverage   79.64%   73.62%   -6.03%     
==========================================
  Files         107       72      -35     
  Lines       12658     8243    -4415     
==========================================
- Hits        10082     6069    -4013     
+ Misses       2576     2174     -402     

Impacted Files	Coverage Δ
alibi/explainers/tests/utils.py
alibi/tests/test_saving.py
alibi/confidence/tests/test_trustscore.py
alibi/utils/tests/test_gradients.py
alibi/explainers/tests/test_anchor_text.py
alibi/tests/utils.py
alibi/api/tests/test_interfaces.py
alibi/tests/test_utils.py
alibi/explainers/tests/test_anchor_tabular.py
alibi/prototypes/tests/test_utils.py
... and 25 more