Set carryforward flag to true on codecov configuration (#3941)

Set carryforward flag to true to past coverage for tests that
are not run on current commit.
You can refer to https://docs.codecov.com/docs/carryforward-flags to
learn more details.

Signed-off-by: Lan Luo <luola@vmware.com>

codecov.yaml

@@ -9,6 +9,7 @@ comment:
   require_base: no
   require_head: no
   after_n_builds: 1
+  show_carryforward_flags: true
 
 coverage:
   status:
@@ -36,6 +37,10 @@ coverage:
         flags:
           - mc-integration-tests
 
+flag_management:
+  default_rules:
+    carryforward: true
+
 ignore:
   - "**/testing/mock_*.go"
   - "**/mock_*.go"