Fix ProcessHelperTest timing issues by using long-running processes

Fixed two failing tests that were using 'echo' commands which complete
almost instantly, making it impossible to verify process running state:

1. test_startProcess_replaceExistingProcess:
   - Changed from 'echo' to 'sleep 10' commands
   - Added proper wait time before checking process state
   - Added verification that only one process exists for the session

2. test_destroyProcess_withRunningProcess:
   - Changed from 'echo' to 'sleep 10' command
   - Simplified polling logic - just wait 100ms for process to start
   - Updated exit code assertion (forcibly destroyed processes may have non-zero exit codes)

These changes ensure the tests can reliably verify that processes are running
before attempting to check their state or destroy them.

Author: claude