net/http2: wait for test goroutines to complete
net/http2: wait for test goroutines to complete

Don't assume goroutines are gone immediately. Any error that occurs will
cause a panic after the test completes.

Fixes golang/go#41299

Change-Id: I00d3e246fd0b265a33a5dc1a536a33866df30d75
Reviewed-on: https://go-review.googlesource.com/c/net/+/254278
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Reviewed-by: Chao Xu <cbdffa0c689f2fb8ee77f97ca085a8c336fd62fc@google.com>
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Trust: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net