tests: cmdline/repl_lock.py and repl_cont.py intermittent failures #18868

Two REPL tests fail intermittently on CI:

cmdline/repl_lock.py — fails on QEMU ARM and RISCV64. The expected
output shows >>> micropython.heap_lock() but the actual output drops
the >>> prompt prefix. Observed 3 times in 20 runs with logs. This is
a REPL prompt timing issue under QEMU emulation.

cmdline/repl_cont.py — fails on macOS. Differences in quote escaping
in REPL continuation prompts ("'" vs '\''). Observed once in 20 runs.
The macOS job has historically been the second most failure-prone job
(4.3% failure rate, 25 failures over 14 months) with all failures
attributed to REPL-related issues. The August 2025 spike (11 macOS
failures) correlates with the GitHub Actions macOS 15 runner migration.

PR #18861 now ignores these failures in CI.

See analysis: https://gist.github.com/andrewleech/5686ed5242e0948d8679c432579e002e