Add a test case for advanced gdb-based debugging of real-time processes.
It currently checks three aspects:
 - Do we resume an RT thread in primary mode after hitting a breakpoint
   in that mode?
 - Do we stop all RT threads immediately when one process thread hits a
   breakpoint?
 - Do we resume all threads then synchronously (at when running on the
   same CPU) when the program continues?

The test first forks a gdb instance that will start the test application
again, but with different command line options so that the second run
executes the inner test logic. The original instance controls gdb via
local pipes and evaluates the debugger output.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>