Resolve TypeScript build error

Merging [9c74f77][] introduced a TypeScript build error, which is
failing our CI build. This commit replaces the old `formSubmitted`
helper with the newer `formSubmitEnded`.

[9c74f77]: 9c74f77#diff-225e8f674fa1e4fd108c22a917fd5673e51909d4a5d8958e4874ae9e402dca2f

src/tests/functional/form_submission_tests.ts

@@ -584,14 +584,14 @@ export class FormSubmissionTests extends TurboDriveTestCase {
     await this.clickSelector('#dialog-formmethod-turbo-frame [formmethod="dialog"]')
     await this.nextBeat
 
-    this.assert.notOk(await this.formSubmitted)
+    this.assert.notOk(await this.formSubmitEnded)
   }
 
   async "test form submission targetting frame skipped within method=dialog"() {
     await this.clickSelector('#dialog-method-turbo-frame button')
     await this.nextBeat
 
-    this.assert.notOk(await this.formSubmitted)
+    this.assert.notOk(await this.formSubmitEnded)
   }
 
   async "test form submission targetting frame skipped with submitter formmethod=dialog"() {