adds test coverage support

.coveragerc

@@ -0,0 +1,5 @@
+[run]
+omit =
+  */_build_ffi.py
+  */_build_wolfssl.py
+  */_ffi.py

Makefile

@@ -60,8 +60,8 @@ test-all: ## run tests on every Python version with tox
 
 check-all: test-all ## run tests on every Python version with tox
 
-coverage: ## check code coverage quickly with the default Python
-	coverage run --source wolfcrypt -m pytest
+cov: install ## check code coverage quickly with the default Python
+	py.test --cov-config .coveragerc --cov=wolfcrypt tests
 	coverage report -m
 	coverage html
 	$(BROWSER) htmlcov/index.html