pkg/runtest: run Linux tests on CI

[ramosian-glider]

Is your feature request related to a problem? Please describe. We currently have a bunch of ARM64 SyzOS tests in sys/linux/test. I run them locally using syz-manager -mode run-tests, but want to continuously run them somewhere to detect regressions.

Describe the solution you'd like Because the manager already has all the building blocks to execute those tests, I was thinking about adding a config option that would run the tests at manager startup and report their failures as bugs.

This can be very helpful for verifying the behavior of SyzOS guest code, but can be also used to validate pseudo-syscalls that can silently stop working now.

An alternative approach would be to add a separate Github CI test target that would effectively run syz-manager -mode run-tests.

Additional context In general, tests in sys/linux/tests do not always validate any functionality. In certain modes, some syscalls may fail leading to test failures. So we'll need to split off the tests meant to be run continuously.

[dvyukov]

Related #4022, #604, #603