Global timeout for the Reframe run

[ekouts]

We would like to configure a time limit for an entire ReFrame run. If this time limit is reached, ReFrame should gracefully terminate all running tests and generate the JSON report, marking any incomplete tests as aborted or failed.

This would help in scenarios where the system is heavily loaded. Currently, our CI system may terminate the ReFrame job while tests are still running. In such cases, we lose all test results, as the JSON report is never generated.

This request is similar to #3187, but it can cover more cases.