x/net/http2: frequent failures in TestClientConnCloseAtBody [1.16 backport]

[gopherbot]

@neild requested issue #49335 to be considered for backport to the next 1.16 minor release.

@gopherbot Please open backport issues for 1.16 and 1.17. Test-only fix for flaky failures.

[gopherbot]

Change https://golang.org/cl/368554 mentions this issue: [internal-branch.go1.16-vendor] http2: don't count aborted streams as active in tests

[mknyszek]

Approving because it's a test-only fix that improves the reliability of the release builders.

CC @dmitshur

[gopherbot]

Change https://golang.org/cl/368594 mentions this issue: [release-branch.go1.16] net/http: update bundled golang.org/x/net/http2