test_sharded_ingest timeouts during pageserver shutdown

[jcsp]

Example:
https://neon-github-public-dev.s3.amazonaws.com/reports/pr-9678/11802652055/index.html#suites/c4b5b5cc329a950d1fae768ab6cbdaf5/6f2d1e56efd29eba

The backtrace in test code indicates that it's setting immediate=True which should be a SIGKILL, but the pageserver logs look like a graceful shutdown and it reports getting a SIGTERM:

2024-11-12T18:53:55.701009Z  INFO Got signal SIGTERM. Terminating gracefully in fast shutdown mode

This test might be exposing a bug in how we do shutdowns in test teardown?