fix(tests): convert ZMK_CONFIG to an absolute path

Zephyr 2.4.0 requires ...
  set(DTS_ROOT)
  list(APPEND DTS_ROOT)
... to use absolute paths.
And therefore ZMK_CONFIG as well.

See: zephyrproject-rtos/zephyr#28180
PR: #467

app/run-test.sh

@@ -27,7 +27,7 @@ fi
 testcase="$path"
 echo "Running $testcase:"
 
-west build -d build/$testcase -b native_posix -- -DZMK_CONFIG=$testcase > /dev/null 2>&1
+west build -d build/$testcase -b native_posix -- -DZMK_CONFIG="$(pwd)/$testcase" > /dev/null 2>&1
 if [ $? -gt 0 ]; then
 	echo "FAIL: $testcase did not build" >> ./build/tests/pass-fail.log
 	exit 1