net/http: test for racing idle conn closure and new requests

TestTransportRemovesH2ConnsAfterIdle is experiencing flaky
failures due to a bug in idle connection handling.
Upon inspection, TestTransportRemovesH2ConnsAfterIdle
is slow and (I think) not currently testing the condition
that it was added to test.

Using the new synctest package, this CL:

- Adds a test for the failure causing flakes in this test.
- Rewrites the existing test to use synctest to avoid sleeps.
- Adds a new test that covers the condition the test was
  intended to examine.

The new TestTransportIdleConnRacesRequest exercises the
scenario where a never-used connection is closed by the
idle-conn timer at the same time as a new request attempts
to use it. In this race, the new request should either
successfully use the old connection (superseding the
idle timer) or should use a new connection; it should not
use the closing connection and fail.

TestTransportRemovesConnsAfterIdle verifies that
a connection is reused before the idle timer expires,
and not reused after.

TestTransportRemovesConnsAfterBroken verifies
that a connection is not reused after it encounters
an error. This exercises the bug fixed in CL 196665,
which introduced TestTransportRemovesH2ConnsAfterIdle.

For #70515

Change-Id: Id23026d2903fb15ef9a831b2df71177ea177b096
Reviewed-on: https://go-review.googlesource.com/c/go/+/631795
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>

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