[lldb][trace] Ensure ProcessTrace plugin can be re-registered (#188336)
Initialize makes sure that it calls RegisterPlugin only once, but
Terminate always calls UnregisterPlugin. This is a problem for tests
that call Initialize/Terminate before and after each test case: the
second case will fail because the trace plugin won't be loaded.
This fixes a test failure introduced by #187768, which adds a test case
that passes on its own but fails when run after the previous test case.