Fix assert in scope test (#2323)

## Checklist

- [-] I have added
[tests](https://www.cursorless.org/docs/contributing/test-case-recorder/)
- [-] I have updated the
[docs](https://github.com/cursorless-dev/cursorless/tree/main/docs) and
[cheatsheet](https://github.com/cursorless-dev/cursorless/tree/main/cursorless-talon/src/cheatsheet)
- [-] I have not broken the cheatsheet

Author: pokey

packages/cursorless-vscode-e2e/src/suite/scopes.vscode.test.ts

@@ -98,7 +98,7 @@ async function runTest(file: string, languageId: string, facetId: string) {
     .replaceAll("\r\n", "\n");
   const delimiterIndex = fixture.match(/^---$/m)?.index;
 
-  assert.isNotNull(
+  assert.isDefined(
     delimiterIndex,
     "Can't find delimiter '---' in scope fixture",
   );