testing: set revive to treat new check failures as errors

Previously, revive was configured to only warn on detected issues.  We
have eliminated all those issues and so can set revive to error out and
thus the 'make check' rule will fail if new issues are introduced.

Signed-off-by: John Mulligan <jmulligan@redhat.com>

.revive.toml

@@ -1,6 +1,6 @@
 
 ignoreGeneratedHeader = false
-severity = "warning"
+severity = "error"
 confidence = 0.8
 errorCode = 1
 warningCode = 0