test: stop node before calling assert_start_raises_init_error

Summary:
...in feature_pruning.
Also add an assert to assert_start_raises_init_error that the node is
not already running.

The change in feature_coinstatsindex test is not relevant since we bypassed that assert_start_raises_init_error by squashing two pull requests in D17714

This is a backport of core#25034

Test Plan: ninja check-functional-extended

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17919