docs: ✏️ byTestId DOM selector

[chimurai]

✅ fixes: #380

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngneat/spectator/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[x] Other... Please describe: Add missing `byTestId` unit tests

What is the current behavior?

byTestId is not documented: https://ngneat.github.io/spectator/docs/queries#dom-selector

What is the new behavior?

• Documented byTestId
• Document how to use byTestId with custom testIdAttribute
  (https://testing-library.com/docs/queries/bytestid#overriding-data-testid)
• Add missing byTestId unit tests
• Updated byTestId documentation for AI assisted coding: https://context7.com/ngneat/spectator

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

byTestId was added in #32

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