[ulib][trace-vthread] Convert tests to use zxtest instead of unittest

This was mostly automated, with some manual changes:
 * change helper functions: return void not bool; calls use ASSERT_NO_FATAL_FAILURES()

Bug: 51652
Test: runs on CQ, but tested locally with:
  fx set bringup.x64 --with-base //garnet/packages/tests:zircon --with-base //bundles/bringup:tests
  fx build
  fx qemu -k -c zircon.autorun.boot=/boot/test/trace-vthread-test-test
Change-Id: I8b921b5f96c2ad236e9d9a548d389de254cdf07a
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/405391
Reviewed-by: George Kulakowski <kulakowski@google.com>
Testability-Review: George Kulakowski <kulakowski@google.com>
Commit-Queue: Mark Seaborn <mseaborn@google.com>