tests: check http/2 and http/3 server responsiveness

Check responsiveness of http/3 server when running.

Also, a test case with http/2 or http/3 server requirement
now implicitl

tests: check http/2 and http/3 server responsiveness

Check responsiveness of http/3 server when running.

Also, a test case with http/2 or http/3 server requirement
now implicitly drags in a 'http' server and we need no longer
mention that in testdata.

Closes #15058

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lib: add curl_multi_waitfds

New function call, similar to curl_multi_fdset()

Closes #13135