fix: add helpful message when test case has non-string code/name

[bmish]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request? (put an "X" next to an item)

[ ] Documentation update
[ ] Bug fix (template)
[ ] New rule (template)
[ ] Changes an existing rule (template)
[ ] Add autofix to a rule
[ ] Add a CLI option
[X] Add something to the core
[ ] Other, please explain:

What changes did you make? (Give an overview)

Instead of crashing with an unhelpful message when encountering a test case that is missing code, or has a non-string value for code / name:

text.replace is not a function 
  at ​​​sanitize​​​ ​./node_modules/eslint/lib/rule-tester/rule-tester.js:207

We are adding asserts with helpful output:

AssertionError [ERR_ASSERTION]: Test case must specify a string value for 'code'

AssertionError [ERR_ASSERTION]: Optional test case property 'name' must be a string

As discussed in the issue, this is not a breaking change since any affected test cases are already failing.

Fixes #13917.

Note that separately, I have an RFC open for some additional test validations that are breaking changes: eslint/rfcs#84

Is there anything you'd like reviewers to focus on?

[eslint-github-bot]

Hi @bmish!, thanks for the Pull Request

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[aladdin-add]

LGTM.

[mdjermanovic]

LGTM, thanks!