tools/buildsystem_sanity_check: add check for APPLICATION in tests

This check verifies the APPLICATION variable is not set by tests application Makefiles. These applications should include the common test Makefile, where this variable is automatically set

dist/tools/buildsystem_sanity_check/check.sh

@@ -188,6 +188,19 @@ check_board_insufficient_memory_not_in_makefile() {
         | error_with_message 'Move BOARD_INSUFFICIENT_MEMORY to Makefile.ci'
 }
 
+# Test applications must not define the APPLICATION variable
+checks_tests_application_not_defined_in_makefile() {
+    local patterns=()
+    local pathspec=()
+
+    patterns+=(-e '^[[:space:]]*APPLICATION[[:space:]:+]=')
+
+    pathspec+=('tests/**/Makefile')
+
+    git -C "${RIOTBASE}" grep "${patterns[@]}" -- "${pathspec[@]}" \
+        | error_with_message "Don't define APPLICATION in test Makefile"
+}
+
 error_on_input() {
     ! grep ''
 }
@@ -200,6 +213,7 @@ all_checks() {
     check_cpu_cpu_model_defined_in_makefile_features
     check_not_setting_board_equal
     check_board_insufficient_memory_not_in_makefile
+    checks_tests_application_not_defined_in_makefile
 }
 
 main() {