tests: kernel: mslab: extend k_mem_slab_alloc() timeout

Extend timeout on k_mem_slab_alloc() for kernel.memory_slabs
memory_slab_1cpu.mslab test suite as a workaround to allow its run
on slow platforms, e.g. simulated intel_ish_5_8_0.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>