tests/avocado: Raise timeout for boot_linux.py:BootLinuxPPC64.test_pseries_tcg

tests/avocado: Raise timeout for boot_linux.py:BootLinuxPPC64.test_pseries_tcg

On my machine, a debug build of QEMU takes about 260 seconds to
complete this test, so with the current timeout value of 180 seconds
it always times out.  Double the timeout value to 360 so the test
definitely has enough time to complete.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20221110142901.3832318-1-peter.maydell@linaro.org>
Message-Id: <20221117172532.538149-9-alex.bennee@linaro.org>