tests: kernel/smp: more timeout

The concurrency and switching tests take quite some time to
finish (relatively speaking). When running in emulator and
under a heavily loaded system (e.g. twister-ing), it may
time out as the emulator is not getting much CPU time. So
make the base timeout longer, which can then be further
adjusted by the board timeout multiplier.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>