arm64: spe: Add timeout to the buffer tests

The SPE buffer tests profile memory accesses and check that the buffer
management interrupt is asserted as expected. However, if the buffer
management interrupt is never asserted (for example, because there's a bug
in KVM's SPE implementation), the test never finishes. Let's add a timeout
to the tests, after which the test is marked as failed if the interrupt was
not asserted.