service/s3: Update unit test to clarify implemented behavior (#4033)

Updates Amazon S3 Output ARN unit test to more specifically describe the
behavior that is being tested.

If the client's resolved endpoint's signing region does not match the
Output ARN's region component the client will return the region not
match error. If the client's resolved endpoint's signing region does
match the Output ARN's region component, but the API client's configured
region includes FIPS, the FIPS configured error will be returned.

service/s3/endpoint_test.go

@@ -265,14 +265,22 @@ func TestEndpoint(t *testing.T) {
 			expectedSigningName:   "s3-outposts",
 			expectedSigningRegion: "us-gov-east-1",
 		},
-		"Outpost AccessPoint FIPS client region": {
-			bucket: "arn:aws-us-gov:s3-outposts:us-gov-east-1:123456789012:outpost:op-01234567890123456:accesspoint:myaccesspoint",
+		"Outpost AccessPoint FIPS client region, resolved signing region does not match ARN region": {
+			bucket: "arn:aws-us-gov:s3-outposts:us-gov-unknown-1:123456789012:outpost:op-01234567890123456:accesspoint:myaccesspoint",
 			config: &aws.Config{
 				EndpointResolver: endpoints.AwsUsGovPartition(),
-				Region:           aws.String("fips-us-gov-east-1"),
+				Region:           aws.String("fips-us-gov-unknown-1"),
 			},
 			expectedErr: "ConfigurationError: client region does not match provided ARN region",
 		},
+		"Outpost AccessPoint FIPS client region, resolved signing region does match ARN region": {
+			bucket: "arn:aws-us-gov:s3-outposts:us-gov-west-1:123456789012:outpost:op-01234567890123456:accesspoint:myaccesspoint",
+			config: &aws.Config{
+				EndpointResolver: endpoints.AwsUsGovPartition(),
+				Region:           aws.String("fips-us-gov-west-1"),
+			},
+			expectedErr: "use of ARN is not supported when client or request is configured for FIPS",
+		},
 		"Outpost AccessPoint FIPS client region with matching ARN region": {
 			bucket: "arn:aws-us-gov:s3-outposts:us-gov-east-1:123456789012:outpost:op-01234567890123456:accesspoint:myaccesspoint",
 			config: &aws.Config{