KEP-2133: add beta test plan

Signed-off-by: Andrew Sy Kim <[email protected]>

Author: andrewsykim

keps/sig-node/2133-kubelet-credential-providers/README.md

@@ -325,6 +325,10 @@ Alpha:
 * unit tests for the exec plugin provider
 * unit tests for API validation
 
+Beta:
+* integration or e2e tests with at least one working plugin implementation
+* unit tests for new concurrency/caching improvements.
+
 ### Graduation Criteria
 
 ### Alpha