[Tests] waitForThrow should diff strings (#26568)

Currently, `waitForThrow` tries to diff the expected value against the
thrown value if it doesn't match. However if the expectation is a
string, we are not diffing against the thrown message. This commit makes
it so if we are matching against message we also diff against message.

packages/internal-test-utils/ReactInternalTestUtils.js

@@ -141,10 +141,18 @@ export async function waitForThrow(expectedError: mixed): mixed {
  typeof expectedError === 'string' &&
  typeof x === 'object' &&
  x !== null &&
- typeof x.message === 'string' &&
- x.message.includes(expectedError)
+ typeof x.message === 'string'
  ) {
- return x;
+ if (x.message.includes(expectedError)) {
+ return x;
+ } else {
+ error.message = `
+Expected error was not thrown.
+
+${diff(expectedError, x.message)}
+`;
+ throw error;
+ }
  }
  error.message = `
 Expected error was not thrown.