Fix flakiness of Verilator debug test in CI

[raylau1]

The Verilator debug test has intermittent timeouts which were not identified as errors due to a mistake in the test script.

Examples:

2026-07-15: https://github.com/lowRISC/mocha/actions/runs/29402763888/job/87311320619
Silent timeout
2026-07-15: https://github.com/lowRISC/mocha/actions/runs/29404079750/job/87315625900
Silent timeout
2026-07-13: https://github.com/lowRISC/mocha/actions/runs/29268618032/job/86880723612
Silent timeout
2026-07-13: https://github.com/lowRISC/mocha/actions/runs/29268585613/job/86880569783
Successful with unexpected log:

jtag0: Rejecting additional connection

2026-07-10: https://github.com/lowRISC/mocha/actions/runs/29079384100/job/86318761150
Successful with unexpected log:

Error: timed out while waiting for target halted
TARGET: riscv-cheri.cpu - Not halted
in procedure 'ocd_gdb_restart'

2026-07-09: https://github.com/lowRISC/mocha/actions/runs/29008488739/job/86086202687
Silent timeout
2026-07-09: https://github.com/lowRISC/mocha/actions/runs/29007257100/job/86082071894
Silent timeout
2026-07-08: https://github.com/lowRISC/mocha/actions/runs/28948578282/job/85889262401
Silent timeout
2026-07-07: https://github.com/lowRISC/mocha/actions/runs/28796060276/job/85839102057
Silent timeout
2026-07-01: https://github.com/lowRISC/mocha/actions/runs/28502548329/job/84483321134
Silent timeout

The suspected cause is a conflict between multiple debug test instances on the same machine, due to multiple virtual JTAG devices or network port conflict.