Fix libtest-json test

Author: Zoxc

src/test/run-make-fulldeps/libtest-json/Makefile

@@ -6,7 +6,7 @@ OUTPUT_FILE := $(TMPDIR)/libtest-json-output.json
 
 all:
 	$(RUSTC) --test f.rs
-	$(call RUN,f) -Z unstable-options --test-threads=1 --format=json > $(OUTPUT_FILE) || true
+	RUST_BACKTRACE=0 $(call RUN,f) -Z unstable-options --test-threads=1 --format=json > $(OUTPUT_FILE) || true
 
 	cat $(OUTPUT_FILE) | "$(PYTHON)" validate_json.py