Increase RPC timeout for the feature_assumevalid test

Summary: This is expected to avoid flakiness when it runs with TSAN.

Test Plan:
With TSAN:

./test/functional/test_runner.py feature_assumevalid.

Run the TSAN build on CI.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5039