fix: assert matches more than the template on multiple CDK copies (a…

…ws#14544)

The `assert` library sometimes fails to get the template out of a
`CloudFormationStackArtifact` if there are multiple copies of the CDK in
the dependency tree.

Fix by replacing an `instanceof` with a duck-type check.

Fixes aws#14468.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/assert-internal/lib/synth-utils.ts

@@ -18,7 +18,7 @@ export class SynthUtils {
    */
   public static toCloudFormation(stack: core.Stack, options: core.SynthesisOptions = { }): any {
     const synth = this._synthesizeWithNested(stack, options);
-    if (synth instanceof cxapi.CloudFormationStackArtifact) {
+    if (isStackArtifact(synth)) {
       return synth.template;
     } else {
       return synth;
@@ -85,3 +85,7 @@ export interface SubsetOptions {
    */
   resourceTypes?: string[];
 }
+
+function isStackArtifact(x: object): x is cxapi.CloudFormationStackArtifact {
+  return 'template' in x;
+}