tests: convert uvm lifecycle dimension from stringly-typed to enum

Replace the stringly-typed uvm_lifecycle with a VmLifecycle str-enum.
Keep pytest IDs and dynamic fixture names unchanged by using enum values
explicitly, and update the A/B fixture to compare enum members.

This makes the dimension states explicit without changing the generated
test matrix.

Signed-off-by: James Curtis <jxcurtis@amazon.co.uk>