Fix UnwindTest repeatability.

- Rewrite some of the UnwindTest tests to properly wait for the process
  to be ready.
- Add a TestScopedPidReaper to make sure that fork process get killed even
  if the test fails. Add this to all tests that fail.
- Create a quiesce function to be used by all of the tests that will
  wait after attaching to a process.

Bug: 65287279

Test: Ran unit tests on hikey960 board and on host repeatedly.
Change-Id: I57084120396f34d8dfb852f3d814bef2056f1b54