tests: basic periph timer

[smlng]

This PR is a refined and stripped down version of #30, i.e. adapting tests scripts and firmware to adhere with the structure and concept of existing tests.

[smlng]

@MichelRottleuthner as discussed f2f these are my refinements of your initial PR which you can use as starting point to work on more complex timer tests.

[MrKevinWeiss]

I tested and it passes on the nucleo-l433rc. Generally I think this is OK assuming it is part of an iteration.

[smlng]

This one should be ready to go. Please note only the new boards support all features, i.e. the debug pins - so failing tests are expected. Alternative would be to remove the tests that require debug pins.

[MichelRottleuthner]

Just tested it again locally and it works as expected also on my nucleo-l476rg.
Two things as a sidenote:
you know whats going on with the nucleo-f767zi (HIL reports timeouts..)?
Why is the extension *.txt used for keyword definition files instead of *.robot? This way we don't get syntax highlighting on Github . For robot both should be fine, no?
Regarding your point on "failing tests are expected" I'm for leaving in the debug-pin related things so we have a strong motivation to properly connect up all the boards soon ;)