Add test cases for SECTION UNION defining multiple identical labels (#1349)

Exported labels should fail to link; non-exported ones should be okay.

test/link/unmangle.cpp

@@ -0,0 +1,16 @@
+#include <stdio.h>
+
+/**
+ * MinGW mangles path names before passing them as command-line arguments.
+ * Some RGBLINK warning/error messages include those mangled paths on Windows.
+ * We need to see those mangled paths in test.sh to replace them with placeholders.
+ * This tool simply echoes each argument, which will be mangled iff they are paths.
+ * (For example, the "/tmp/foo" will be unmangled to something like
+ * "C:/Users/RUNNER~1/AppData/Local/Temp/foo".)
+ */
+
+int main(int argc, char *argv[]) {
+	for (int i = 1; i < argc; i++)
+		puts(argv[i]);
+	return 0;
+}