[roll] Roll fuchsia [kernel][unittest] Improve 'k ut' repeat option (reduce overheads)

In commit 5fdb997c67e, I added support to 'k ut' to run specific unit
test suites repeatedly; this was helpful for running many instances of
particular tests, to shake out bugs.

Improve the implementation of the repeat option, to reduce overheads:
* k ut runs tests in a dedicated kernel thread, in a new user-marked
  address space. The initial implementation of 'k ut -r' was a loop
  around the outer-most call, which meant that thread creation/destroy
  and address space create/destroy were done for every repeat. This
  overhead added up, particularly for short tests.
* Rework printouts, to be less verbose and clearer about which test
  cases are running. (printing has overheads; plus the verbosity of
  the old format made it difficult some times to see timing from
  past runs).

This CR substantially reduces the runtime of repeated tests.

Tested:
* k ut -r 10000000 dpc_tests
  New example print:
  00000:00000>   repeatedly queue the same dpc : PASSED (795499 nSec)  [2021155 / 1100000001]

* k ut -r 10000 all

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/466816
Original-Revision: 2c3e42d5b7213b1b312e9407a6957590539ec7fa
GitOrigin-RevId: b531fb4f120bca1a9ae672ceddf3ed47b4811d11
Change-Id: Id8650fb722fa194f887ecb0f3fbe3715d1ee8c36