congure_mock: add capability to provide actual methods

[miri64]

Contribution description

This extends the congure_mock so that actual congure method drivers can be added, in case the test needs some actual functionality.

Testing procedure

No tests are provided for the new functionality (it will be used in a later PR). tests/congure_test should still compile and pass.

$ BOARD=iotlab-m3 make -C tests/congure_test/ --no-print-directory -j flash test
Building application "tests_congure_test" for "iotlab-m3" with MCU "stm32".

[…]
   text	   data	    bss	    dec	    hex	filename
  18300	    132	   2536	  20968	   51e8	/home/mlenders/Repositories/RIOT-OS/RIOT/tests/congure_test/bin/iotlab-m3/tests_congure_test.elf
For full programmer output add PROGRAMMER_QUIET=0 or QUIET=0 to the make command line.
 ✓ Flashing done! (programmer: 'openocd' - duration: 2.01s)
......................................
----------------------------------------------------------------------
Ran 38 tests in 18.911s

OK

Issues/PRs references

None, but part of the larger congestion control effort.

[fjmolinas]

ACK, tests are green, @miri64 is maintaining this so trusting this is needed.

[miri64]

ACK, tests are green, @miri64 is maintaining this so trusting this is needed.

It is used in #16156 and its follow-ups ;-).