deps: the pins advance, and a missing interpreter skips rather than vanishes

libavr and pureboot move to the tips that register their checks everywhere.
fantemp.reachability stops being wrapped in `if(Python3_FOUND)`: it is
registered on every host now and skips where the interpreter is absent, so what
`ctest -N` counts is a fact about this repository rather than about the machine
that configured it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/CMakeLists.txt

@@ -20,13 +20,15 @@ add_test(NAME fantemp.consteval
 # The board has no reset line and no programming header, so the loader-entry
 # route in the emitted image is the only thing standing between a firmware change
 # and an unreflashable board.
-find_package(Python3 COMPONENTS Interpreter)
-if(Python3_FOUND)
-	add_test(NAME fantemp.reachability
-	         COMMAND ${Python3_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/check_reachability.py
+# A check whose interpreter this host lacks is registered and *skips*, rather
+# than leaving the suite a different size on a different machine - which is a
+# suite nothing can be compared against. The launcher and the marker are
+# libavr's (`cmake/checks.cmake`).
+libavr_launcher(_python "${LIBAVR_NO_PYTHON}" ${Python3_EXECUTABLE})
+add_test(NAME fantemp.reachability
+         COMMAND ${_python} ${CMAKE_CURRENT_SOURCE_DIR}/check_reachability.py
                  --objdump ${CMAKE_OBJDUMP} --elf $<TARGET_FILE:fantemp>
                  --image $<TARGET_FILE_DIR:fantemp>/fantemp.bin
                  --loader-base ${PUREBOOT_BASE_HEX})
-else()
-	message(STATUS "Python not found - the reachability check is skipped")
-endif()
+
+libavr_skip_unverified()