r/aws_iam_role_policy_attachments_exclusive(test): fix test configura…

…tion (#39729)

During a refactor of the initial implementation, the `Config` field was updated during the first test step but not the second. This introduced a race condition between detachment and deletion of the out of band policy, resulting in intermittent failures deleting the policy. The impacted test now appropriately uses the same configuration throughout all test steps.

```console
% make testacc PKG=iam TESTS=TestAccIAMRolePolicyAttachmentsExclusive_
make: Verifying source code with gofmt...
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go1.23.2 test ./internal/service/iam/... -v -count 1 -parallel 20 -run='TestAccIAMRolePolicyAttachmentsExclusive_'  -timeout 360m
2024/10/15 13:56:16 Initializing Terraform AWS Provider...

--- PASS: TestAccIAMRolePolicyAttachmentsExclusive_empty (15.49s)
--- PASS: TestAccIAMRolePolicyAttachmentsExclusive_disappears_Role (17.15s)
--- PASS: TestAccIAMRolePolicyAttachmentsExclusive_disappears_Policy (17.45s)
--- PASS: TestAccIAMRolePolicyAttachmentsExclusive_basic (17.85s)
--- PASS: TestAccIAMRolePolicyAttachmentsExclusive_outOfBandAddition (25.47s)
--- PASS: TestAccIAMRolePolicyAttachmentsExclusive_outOfBandRemoval (25.58s)
--- PASS: TestAccIAMRolePolicyAttachmentsExclusive_multiple (25.99s)
PASS
ok      github.com/hashicorp/terraform-provider-aws/internal/service/iam        31.059s
```

internal/service/iam/role_policy_attachments_exclusive_test.go

@@ -273,7 +273,7 @@ func TestAccIAMRolePolicyAttachmentsExclusive_outOfBandAddition(t *testing.T) {
 				ExpectNonEmptyPlan: true,
 			},
 			{
-				Config: testAccRolePolicyAttachmentsExclusiveConfig_basic(rName),
+				Config: testAccRolePolicyAttachmentsExclusiveConfig_outOfBandAddition(rName, oobPolicyName),
 				Check: resource.ComposeTestCheckFunc(
 					testAccCheckRoleExists(ctx, roleResourceName, &role),
 					testAccCheckRolePolicyAttachmentsExclusiveExists(ctx, resourceName),