tests: bluetooth: bsim_test_multiple: Increase test timeout

This commit increases the timeout value for the `bsim_test_multiple`
BSIM test because it may take longer to complete with the
parallelisation enabled due to the context switching overhead and the
overallocation of the CPU cores.

Signed-off-by: Stephanos Ioannidis <stephanos.ioannidis@nordicsemi.no>