[webview_flutter] Adjust test URLs again

[stuartmorgan-g]

These tests are once again having issues, suggesting that example.com
may not have been a good choice of URL. Instead use the google.com
robots page, which presumably isn't subject to bot redirection like the
original problem that led to replacing google.com with example.com.

Also replaces the common test URLs with constants to make switching
again easier, per a comment in the previous PR.

Speculative fix for flutter/flutter#91112

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[cyanglaz]

The meat looks good, 1 test failed tho, probably some typo?