test: throw element not found error

[sun0day]

Description

Sometimes toEl fails to find the element in CI, which will cause el.evaluate throw the error Cannot read properties of null (reading 'evaluate') and the error stack position is incorrect, it's hard to debug which test case fails.

e.g

There is no getBg call in this test case actually 😢

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the PR Title Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[stackblitz]

Run & review this pull request in StackBlitz Codeflow.