Move omit to run section in .coveragerc

Coverage used to be configured to omit certain directories while
reporting.

This commit slightly optimizes coverage to already omit those
directories while measuring coverage.

tests/.coveragerc

@@ -1,15 +1,15 @@
 [run]
 branch = True
 
-[report]
-exclude_lines =
-    pragma: no cover
-    def __str__
-    if __name__ == .__main__.:
-
 omit =
     # Command-line scripts.
     */tuf/scripts/client.py
     */tuf/scripts/repo.py
     */tests/*
     */site-packages/*
+
+[report]
+exclude_lines =
+    pragma: no cover
+    def __str__
+    if __name__ == .__main__.: