[zircon][unit tests] Add unit tests of dpc_queue

Add basic tests that exercise non-blocking and blocking calls from dpc_queue.
Check that a DPC runs on the same CPU that it was queued on in the absence of
hotplug. Also check queueing many DPCs and seeing that they all execute.

Tested:
* k ut dpc_tests on qemu x86, qemu-kvm x86, qemu ARM
Ex (qemu ARM):
$ k ut dpc_tests
[00010.025] 00000:00000> dpc_tests : Running 2 tests...
[00010.030] 00000:00000>   basic test of dpc_queue    : PASSED (3393392 nSec)
[00010.037] 00000:00000>   test queueing lots of DPCs : PASSED (6933168 nSec)
[00010.037] 00000:00000> dpc_tests : All tests passed (2/2) in 11158208 nSec
[00010.041] 01188:01209>
[00010.041] 01188:01209> SUMMARY: Ran 1 test case: 0 failed
$

Change-Id: I405c9eaf99e9f6050378293889d417e97a54ab81