[kernel][unittest] Don't use computed printf format strings

This code was computing a format string at runtime instead of just
using %-*s.

Test: CQ & manual inspection of kernel test output
Change-Id: Ided3081f753376964d66648b2caaa7e3e24afdbb

[kernel] Add unit test for MessagePacket

Move UserMemory test helper out of mbuf_tests.cpp so it can be reused.

ZX-1881 #done

Change-Id: I89a3683f284ce26046ea49b9010e6173c709fbc1

[kernel] Run unittests with user address space

Update mbuf_tests.cpp to use user memory insetad of fake_user_ptr.

Remove fake_user_ptr and testable_user_ptr.

Convert unittest.c to C++.

Tested this change by running kernel unittests via "k ut all" and
kernel.shell

ZX-1914 #done

Change-Id: I9898cb23da109485dee6db06de11cbd0928a6ba9

[kernel] enable stricter printf warnings

Change-Id: I02347b0716cec4e3e147318d37415d194b0affe4

[kernel] Begin cleanup of kernel unit tests

Setup the framework and port some tests to this new framework

Change-Id: I09e1d78095d3de582c74c71e0dd1583706696b24

[magenta] Initial commit