test/crimson/seastore: exercise the failsafe-full threshold

Add storage_full_failsafe_threshold to the RBM-backed transaction
manager tests: a fresh store reports not-full at the default ratio,
real extent allocations flip is_storage_full() once the used fraction
crosses osd_failsafe_full_ratio, and restoring the default ratio
clears the condition, verifying the threshold tracks the config knob.

Signed-off-by: Shai Fultheim <shai.fultheim@gmail.com>