crypto/tls: move shared code and resources to handshake_test.go

crypto/tls: move shared code and resources to handshake_test.go

Removed cross-dependencies between handshake_server_test.go and
handshake_client_test.go; moved all initialization to TestMain; replaced
SSLKEYLOGFILE environment variable with -keylog flag.

Change-Id: Ida6712daa44e01a2c00658e8a1896087ee88bcb0
Reviewed-on: https://go-review.googlesource.com/c/go/+/183057
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>