improve output of C tests

integration-tests/src/bindings_tests/test.c

@@ -1,6 +1,30 @@
+#include <stdio.h>
 #include "unity.h"
 
 // these functions are shared between all test "suites"
 // and cannot be customized per-suite.
 void setUp(void) { }
 void tearDown(void) { }
+
+static FILE *output = NULL;
+
+// Set up for test_output, writing output to "TEST-OUTPUT" in the
+// current directory.  The Rust test harness reads this file to get
+// the output and display it only on failure.  This is called by
+// the Rust test harness
+void setup_output(void) {
+    output = fopen("TEST-OUTPUT", "w");
+}
+
+// Close the output file.  Called by the Rust test harness.
+void finish_output(void) {
+    fclose(output);
+    output = NULL;
+}
+
+// this replaces UNITY_OUTPUT_CHAR, and writes output to
+// TEST-OUTPUT in the current directory; the Rust test harness
+// will read this data if the test fails.
+void test_output(char c) {
+    fputc(c, output);
+}