Fix typo in test cleanup: replace pgrep -9 with pkill -9

[loplex]

The cleanup logic was attempting to use pgrep with the -9 flag, which is invalid. This switches the command to pkill to correctly send SIGKILL to remaining child processes.

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[Copilot]

Pull request overview

This PR fixes a bug in the test cleanup logic where the wrong command was being used to kill remaining child processes. The code was attempting to use pgrep -9, but pgrep is a process search tool that doesn't accept signal flags like -9 (SIGKILL). The fix correctly changes this to pkill -9, which properly sends SIGKILL to terminate any remaining child processes after the initial SIGTERM attempt.

Key Changes:

• Corrected command from pgrep to pkill in the cleanup logic to properly send SIGKILL signal

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[MikeMcQuaid]

Thanks for the fix!