Stabilize compile kill-compilation test

[emil-e]

Summary

• keep the kill-compilation regression test focused on buffer discovery
• mock interrupt-process so the test no longer depends on OS signal delivery and sentinel timing
• move the spawned process teardown into unwind-protect cleanup

Verification

• emacs --batch -Q -L lisp -L test -l ert -l test/ghostel-test-helpers.el -l test/ghostel-compile-test.el --eval "(ert-run-tests-batch-and-exit 'ghostel-test-compile-kill-compilation-finds-live-buffer)"
• git diff --check