test: better snapshot errors

[wjhsf]

Details

If you have a test fixture for a "success" scenario, it will generate an expected.html (or .js, etc) with content and an empty error.txt. If you then update some code and it causes a failure, then the test output will be an empty expected.html (or .js, etc) and an error.txt with content. The snapshot checker only compares files of the same name, so the output looks lomething like this:

 FAIL |lwc-ssr-compiler|  src/__tests__/fixtures.spec.ts > fixtures > wire/errors/throws-when-wired-prop-method-collision/index.js
Error: SSR Fixture `fixtures > wire/errors/throws-when-wired-prop-method-collision/index.js 1` mismatched

- Expected
+ Received

+ instance.wired is not a function

That's not very helpful, so I added some logic to check for that case (and the other way around), to make it more clear that your outputs have gotten mixed up.

 FAIL |lwc-ssr-compiler|  src/__tests__/fixtures.spec.ts > fixtures > wire/errors/throws-when-wired-prop-method-collision/index.js
AssertionError: Expected content in expected.html, but found content in error.txt.

- Expected
+ Received

- <x-wire>
-   <template shadowrootmode="open">
-     <div>
-       Wired? true
-     </div>
-     <div>
-       Using method? false
-     </div>
-   </template>
- </x-wire>
+ instance.wired is not a function

Does this pull request introduce a breaking change?

• 😮‍💨 No, it does not introduce a breaking change.
• 💔 Yes, it does introduce a breaking change.

Does this pull request introduce an observable change?

• 🤞 No, it does not introduce an observable change.
• 🔬 Yes, it does include an observable change.

GUS work item

[nolanlawson]

Very nice!

[nolanlawson]

It's a shame we have to keep reading in snapshot files when presumably Vitest is already doing this elsewhere.

[nolanlawson]

TIL about AssertionError!