[ResponseOps][Cases] Fix custom fields flaky test.

[adcoelho]

fixes #188133

Summary

I simplified the failing test a bit to make it less likely to timeout.

1. We checked for the test and the id to make sure the component was rendered, it was redundant.
2. We looked for all custom fields(4) and I reduced the check to just 2(which have different types).
3. Updated another test that looked for a label to look for an id(for consistency).

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[elasticmachine]

Pinging @elastic/response-ops-cases (Feature:Cases)

[kibana-ci]

💚 Build Succeeded

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• Commit: e2016b3

Metrics [docs]

✅ unchanged

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cc @adcoelho