fix(ci): gave the unit tests 120 seconds to finish in CI instead of 60

[FlareCoding]

Summary

• The aarch64 unit tests run under emulation on GitHub's runners and recently took between 27 and 63 seconds, so a slow runner could hit the 60 second limit with every test passing.
• CI gives the test run 120 seconds, and local runs keep the 60 second default.

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Note

Low Risk
CI-only workflow env change; no production or test logic changes beyond allowing longer QEMU runs.

Overview
CI kernel unit-tests now set STLX_TEST_TIMEOUT=120 on the make test step so QEMU runs get 120 seconds instead of the harness default of 60 (scripts/run_tests.sh uses STLX_TEST_TIMEOUT with a 60s fallback).

This targets flaky CI failures on slow runners—especially aarch64 under emulation, where recent runs were already near the old limit while tests still passed. Local make test behavior is unchanged when the env var is unset.

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