lisa.tests.util_tracking: Fix debug plotting
lisa.tests.util_tracking: Fix debug plotting

* Avoid interactive plots as they leak memory
* Remove ax=axis from axvline() call since it's redundant with the method call
  on the axis itself.

credp/lisa,ARM-software/lisa,credp/lisa,ARM-software/lisa,credp/lisa,credp/lisa,ARM-software/lisa,ARM-software/lisa