Fix and re-enable sts_assume_role integration tests (#46026)

* Fix the STS assume role error message assertion when the role to assume does not exist.

test/integration/targets/sts_assume_role/aliases

@@ -1,4 +1,3 @@
 cloud/aws
 shippable/aws/group1
 iam_role
-disabled

test/integration/targets/sts_assume_role/tasks/main.yml

@@ -290,14 +290,14 @@
       assert:
         that:
           - 'result.failed'
-          - "'Not authorized to perform sts:AssumeRole' in result.msg"
+          - "'Access denied' in result.msg"
       when: result.module_stderr is not defined
 
     - name: assert assume not existing sts role
       assert:
         that:
           - 'result.failed'
-          - "'Not authorized to perform sts:AssumeRole' in result.module_stderr"
+          - "'Access denied' in result.module_stderr"
       when: result.module_stderr is defined
 
     # ============================================================