test: Refactor setting of dummy error handlers

Multiple tests need to register a function to quiesce errors from
libvirt when using a connection and doing negative tests. Each of those
tests had a static function to do so. This can be replaced by a utility
function that enables the errors when debug is enabled.

This patch adds virtTestQuiesceLibvirtErrors() and refactors test that
use private handlers.

tests/testutils.h

@@ -69,6 +69,8 @@ unsigned int virTestGetExpensive(void);
 
 char *virtTestLogContentAndReset(void);
 
+void virtTestQuiesceLibvirtErrors(bool always);
+
 int virtTestMain(int argc,
                  char **argv,
                  int (*func)(void));