Add logging for warnings during test suite run

The default configuration for the warnings module changed some time ago
so we lost the ability to detect deprecation warnings and such early.

This commit sets up the test suite to redirect all warnings through our
logging system. Additionally we enable DeprecationWarnings as a default
and throw exceptions on all warnings related to pelican modules.

This enables output of warnings related to dependencies, while letting
tests only fail if the warnings are originating from pelican's source.

Also adding a test to detect if warnings cause an Exception as expected.

pelican/tests/__init__.py

@@ -1,2 +1,15 @@
 import logging
+import warnings
+
+from pelican.log import log_warnings
+
+# redirect warnings modulole to use logging instead
+log_warnings()
+
+# setup warnings to log DeprecationWarning's and error on
+# warnings in pelican's codebase
+warnings.simplefilter("default", DeprecationWarning)
+warnings.filterwarnings("error", ".*", Warning, "pelican")
+
+# Add a NullHandler to silence warning about no available handlers
 logging.getLogger().addHandler(logging.NullHandler())