fix(cdk/testing): allow use of plaintext-only for contenteditable (#29999)

(cherry picked from commit b3c93c5)

Author: tatp22

src/cdk/testing/selenium-webdriver/selenium-web-driver-element.ts

@@ -161,7 +161,11 @@ export class SeleniumWebDriverElement implements TestElement {
   async setContenteditableValue(value: string): Promise<void> {
     const contenteditableAttr = await this.getAttribute('contenteditable');
 
-    if (contenteditableAttr !== '' && contenteditableAttr !== 'true') {
+    if (
+      contenteditableAttr !== '' &&
+      contenteditableAttr !== 'true' &&
+      contenteditableAttr !== 'plaintext-only'
+    ) {
       throw new Error('setContenteditableValue can only be called on a `contenteditable` element.');
     }
 

src/cdk/testing/testbed/unit-test-element.ts

@@ -193,7 +193,11 @@ export class UnitTestElement implements TestElement {
   async setContenteditableValue(value: string): Promise<void> {
     const contenteditableAttr = await this.getAttribute('contenteditable');
 
-    if (contenteditableAttr !== '' && contenteditableAttr !== 'true') {
+    if (
+      contenteditableAttr !== '' &&
+      contenteditableAttr !== 'true' &&
+      contenteditableAttr !== 'plaintext-only'
+    ) {
       throw new Error('setContenteditableValue can only be called on a `contenteditable` element.');
     }