[system][ulib][test-utils] Create test-utils library.

test-utils is moved from unittest to its own library.
The reason for this is that test-utils uses mxio but
unittest cannot use mxio. See system/ulib/unittest/README.md
and system/utest/core/README.md.

Change-Id: Id79ea8a4cf8cc948994cef5bcc3dfe035e90bff4