tests/tcg: Use SIGKILL for timeout

tests/tcg: Use SIGKILL for timeout

linux-user blocks all signals while attempting to handle guest
signals (e.g. ABRT), which means that the default TERM sent by timeout
has no effect -- KILL instead.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230117035701.168514-2-richard.henderson@linaro.org>
[AJB: expanded commit message from cover letter]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230124180127.1881110-17-alex.bennee@linaro.org>