[7.16] Make fixture app wait until network is idle before running authentication tests. (#119715)

[kibanamachine]

Backports the following commits to 7.16:

• Make fixture app wait until network is idle before running authentication tests. (Make fixture app wait until network is idle before running authentication tests. #119715)

[kibana-ci]

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@elasticmachine merge upstream

cc @azasypkin

[azasypkin]

@XavierM thanks for review and merging! Sorry for not making this clear enough in the #119715 (comment) description, but we need to explicitly enable the relevant tests in 7.x branch (they were skipped only there): https://github.com/elastic/kibana/blob/7.16/x-pack/test/security_functional/tests/login_selector/basic_functionality.ts#L20-L21 🙈 Would you mind helping with that?

Note to myself: Re-enable tests in 7.16 backport and run x30 flaky test suite there.