internal/verify(test): add malformed old content test case

Before:

```console
% go test -count=1 ./internal/verify/...
--- FAIL: TestSecondJSONUnlessEquivalent (0.00s)
    json_test.go:405: unexpected error with test case malformed old: parsing policy 1: parsing statement 1: 1 error(s) decoding:

        * '[0].Condition[StringLike]' expected a map, got 'slice'
FAIL
FAIL    github.com/hashicorp/terraform-provider-aws/internal/verify     0.702s
```

After:

```console
% go test -count=1 ./internal/verify/...
ok      github.com/hashicorp/terraform-provider-aws/internal/verify     0.383s
```

internal/verify/json_test.go

@@ -354,6 +354,48 @@ func TestSecondJSONUnlessEquivalent(t *testing.T) {
 			newPolicy: "",
 			want:      "",
 		},
+		{
+			name: "malformed old",
+			oldPolicy: `{
+  "Version": "2012-10-17",
+  "Statement": [
+    {
+      "Effect": "Allow",
+      "Action": [
+        "s3:ListBucket"
+      ],
+      "Condition" : {
+        "StringLike" : ["demo-prefix/"]
+      },
+      "Resource": "*"
+    }
+  ]
+}`,
+			newPolicy: `{
+  "Version": "2012-10-17",
+  "Statement": [
+    {
+      "Effect": "Allow",
+      "Action": [
+        "s3:ListBucket"
+      ],
+      "Resource": "*"
+    }
+  ]
+}`,
+			want: `{
+  "Version": "2012-10-17",
+  "Statement": [
+    {
+      "Effect": "Allow",
+      "Action": [
+        "s3:ListBucket"
+      ],
+      "Resource": "*"
+    }
+  ]
+}`,
+		},
 	}
 
 	for _, v := range testCases {