http2: skip synctest.Wait in testers using a real connection

[Jefftree]

Benchmarks using a real connection are not in a synctest bubble, so greet panicked.

Fixes golang/go#81399

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Patch Set 1:

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[Jefftree]

Closing. From go1.27 the implementation lives in net/http/internal/http2, which does not carry these benchmarks, so this only applies to older builds.