Add "global" timeout

[aviramha]

We often find ourselves adding the same timeout proc macro to each test and would love if we could set it in a global level then override on a function level.
I was thinking of adding RSTEST_TIMEOUT environment variable to control it then logic would be if function timeout exists - use that, if not, try to use RSTEST_TIMEOUT.

I can send a PR if the idea makes sense.

[la10736]

Yes, it have sense.

THX