testing: enable exported vars check as warning

(Re-)enable the revive check for exported vars/functions/etc at the
warning level. We have a lot of pre-existing failures here so we do not
want to fail on these, but we would like a list so we can work on
eliminating them.

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

.revive.toml

@@ -14,7 +14,8 @@ warningCode = 0
 [rule.error-return]
 [rule.error-strings]
 [rule.error-naming]
-#[rule.exported]
+[rule.exported]
+severity = "warning"
 [rule.if-return]
 [rule.increment-decrement]
 #[rule.var-naming]