tests/{cpp_ext, cpp_exclude}: model in kconfig

[aabadie]

Contribution description

This PR slightly adapts the dependencies of tests/cpp_ext and tests/cpp_exclude test application (just remove apparently useless libstdcpp required feature) and add to them a Kconfig app.config.test file.

Testing procedure

Green Murdock should be enough

Issues/PRs references

I dropped that change from #17992 because I couldn't find why libstdcpp dependency resolution is different with and without TEST_KCONFIG set on esp32. And apparently libstdcpp is not needed in these applications.

[aabadie]

This one is finally green. Just missing an ACK

[benpicco]

Looks pretty straightforward

[MrKevinWeiss]

Hmm, the only thing I was unsure about was a module selecting HAS_* which is normally selected based on the arch, for example, the arduino-mega2560 does not have libstdcpp. It passes murdock because the supported boards check is done with make but when running with TEST_KCONFIG we get:

/home/weiss/wd/RIOT/tests/cpp_exclude/main.cpp:21:18: fatal error: cstdio: No such file or directory
compilation terminated.
make[1]: *** [/home/weiss/wd/RIOT/Makefile.base:166: /home/weiss/wd/RIOT/tests/cpp_exclude/bin/arduino-mega2560/application_tests_cpp_exclude/main.o] Error 1
make: *** [/home/weiss/wd/RIOT/tests/cpp_exclude/../../Makefile.include:744: application_tests_cpp_exclude.module] Error 2
make: Leaving directory '/home/weiss/wd/RIOT/tests/cpp_exclude'

[MrKevinWeiss]

I just didn't have a better way to solve it at the moment.