TST: Omit tests folders from coverage

xref #12634

Author: Gábor Lipták <gliptak@gmail.com>

Closes #12721 from gliptak/covomit and squashes the following commits:

8bfbb59 [Gábor Lipták] Omit tests folders from coverage

.coveragerc

@@ -1,6 +1,7 @@
 # .coveragerc to control coverage.py
 [run]
 branch = False
+omit = */tests/*
 
 [report]
 # Regexes for lines to exclude from consideration
@@ -23,4 +24,4 @@ exclude_lines =
 ignore_errors = False
 
 [html]
-directory = coverage_html_report
+directory = coverage_html_report