Add SIGTERM regression test for Hybrid fork interrupt handling

[samuel-williams-shopify]

Summary

Follow-up to #58 / #59.

SIGINT and SIGTERM are intentionally treated as equivalent in this library: both are trapped in each fork and converted into the same Interrupt exception (Forked::Child.fork):

Signal.trap(:INT){::Thread.current.raise(Interrupt)}
Signal.trap(:TERM){::Thread.current.raise(Interrupt)}  # Same as SIGINT.

The fix in #59 (Generic#interrupt sets @stopping = true) therefore covers a single SIGTERM to a Hybrid fork as well — it must drain the inner threads and exit, rather than respawning them forever (the inner container runs with restart: true, the default for async-service managed services).

This PR adds a regression test for the SIGTERM path so the equivalence is locked in and can't silently regress.

Changes

• Refactors the existing single-SIGINT test into a shared helper.
• Exercises both :INT and :TERM via the helper.

Verification

• Both tests pass with the fix.
• Both tests fail when @stopping = true is reverted in Generic#interrupt, confirming they are meaningful regression tests rather than false passes.
• Full suite: 170/170 passing.