tests/avocado: push default timeout to QemuBaseTest

tests/avocado: push default timeout to QemuBaseTest

All of the QEMU tests eventually end up derrived from this class. Move
the default timeout from LinuxTest to ensure we catch them all. We
keep the 15 minute timeout as currently some of the more heavyweight
CFI and TCG tests can overrun. We should aim to drop it down to 2
minutes which is a more reasonable target for tests to aim for but we
want to get this release out.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
[AJB: revert to 15 min timeout for v2]
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220822165608.2980552-2-alex.bennee@linaro.org>