crypto/tls: fix flaky handshake cancellation tests

Simplified both tests significantly by removing logic for writing
the client/server side messages. The flake was likely because of a
race between the closing of the local pipe from inside the test
and closing of the pipe from within the handshakeContext goroutine.
Wait to close the local pipe in the test until after the test
has finished running.

Fixes #45106
Fixes #45299

Change-Id: If7ca75aeff7df70cda03c934fa9d8513276d465d
Reviewed-on: https://go-review.googlesource.com/c/go/+/305250


Trust: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>