quic: fix expected ACK Delay in client's ACK after HANDSHAKE_DONE

testConn.handshake runs through the initial QUIC handshake
and verifies that the connection under test sends an expected
sequence of handshake messages.

The last datagram in the handshake is sent by the client,
and contains an ACK for the last datagram sent by the server.

The client sends this ACK after max_ack_delay (25ms) passes, minus
the timer granularity (1ms). The timer granularity is a constant
containing the expected maximum delay between a timer event's
scheduled time and the timer actually firing.

The expected handshake datagram used by testConn.handshake contains
an ACK with an ACK Delay value of 25ms (max_ack_delay).
This doesn't account for the timer granularity adjustment.
However, since testConn.handshake advances time by 25ms rather than 24ms,
the test connection sends the ACK at the later time and includes a
larger ACK Delay value.

Fix testConn.handshake to sleep for the expected delay (24ms).
Fix the expected handshake datagram accordingly.

This all avoids test failures after switching this package to use
testing/synctest's fake clock, under which the connection sends
this ACK at the scheduled time rather than a time under direct
control of the test.

Change-Id: I1af6e02e02f6493758e41db45a46d06a65441a7b
Reviewed-on: https://go-review.googlesource.com/c/net/+/714480
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>