tlstest_test: Go 1.12 / TLS 1.3 fix
tlstest_test: Go 1.12 / TLS 1.3 fix

TLS 1.3 pushes detection of some client-side handshake errors to the time of
the first read. For more details, see:
https://github.com/golang/go/issues/28779#issuecomment-442694636

Signed-off-by: Alain Jobart <1fe0f83f394665206b2ba043f2198fe03353e5b6@google.com>

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