fix(selectors): reset regex state before matching each element

[Abnoz.v2 (Abnoz01)]

Summary

• getByText, getByLabel and the text-matches engine reuse a single RegExp across elements. With the g or y flag, RegExp.prototype.test carries lastIndex between calls, so every other match was skipped (getByText(/foo/g) found 4 of 6 elements).
• Reset lastIndex before each test, also in the expect text matcher, and add a test.

Fixes #42813

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