Interrupt overhead regression testing (GIT8266O-132)

[rumpeltux]

This pull request adds a test case for a timer interrupt.

It specifically also records the time it takes for the interrupt to enter the handler function and to finish alltogether.
While 913188f simplified the code, it added a significant regression in interrupt overhead cycles, which is problematic for apps with high interrupt load.

The test added in this pull-request is also a prerequisite for measuring improvements:
e.g. in my experimental fast-interrupts branch, the overhead time is reduced to <50% of the current values (d39c3b0: 4.8μs → 2.4μs per interrupt)

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