Fix garbled test output (#18822)

* Fix garbled test output

* Correct SRC listing

builddefs/build_test.mk

@@ -71,7 +71,7 @@ include $(BUILDDEFS_PATH)/build_full_test.mk
 endif
 
 $(TEST)_SRC += \
-	tests/test_common/main.c \
+	tests/test_common/main.cpp \
 	$(QUANTUM_PATH)/logging/print.c
 
 $(TEST_OBJ)/$(TEST)_SRC := $($(TEST)_SRC)

tests/test_common/main.cpp

@@ -5,7 +5,7 @@ extern "C" {
 #include "debug.h"
 
 int8_t sendchar(uint8_t c) {
-    fprintf(stderr, "%c", c);
+    fprintf(stdout, "%c", c);
     return 0;
 }