tls: fix flaky TestHandshakeConnectionCancellations test

[marten-seemann]

Explanation:

go-libp2p/p2p/security/tls/transport_test.go

Lines 289 to 291 in 84ded7d

	// crypto/tls' cancellation logic works by spinning up a separate Go routine that watches the ctx.
	// If the ctx is canceled, it kills the handshake.
	// We need to make sure that the handshake doesn't complete before that Go routine picks up the cancellation.