http2: simplify ClientConn Close and Shutdown tests

Split testClientConnClose into four separate tests,
rather than having one big function which performs
different tests depending on its parameter.

Use the more modern testClientConn in these tests,
for simplicity.

Drop the activeStreams function, which pokes a bit too
far into implementation internals and isn't necessary
for the new tests. (It had one use outside of
testClientConnClose, which provides little useful
signal and can also be dropped.)

Change-Id: Id8d1c7feab59c1f041bc2d1cf0398e8b1e230c69
Reviewed-on: https://go-review.googlesource.com/c/net/+/701005
Reviewed-by: Nicholas Husin <nsh@golang.org>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>