tests/avocado: add explicit timeout for Aarch64 TCG tests

We don't want to rely on the soon to be reduced default time. These
tests are still slow for something we want to run in CI though.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220914155950.804707-4-alex.bennee@linaro.org>

tests/avocado/boot_linux.py

@@ -63,6 +63,7 @@ class BootLinuxAarch64(LinuxTest):
     :avocado: tags=machine:virt
     :avocado: tags=machine:gic-version=2
     """
+    timeout = 240
 
     def add_common_args(self):
         self.vm.add_args('-bios',