Add unit test for tree

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

tests/tree.c

@@ -0,0 +1,19 @@
+#include "check.h"
+
+int main(void)
+{
+	int result = 0;
+	size_t i;
+	struct { char *path; int fail; } arr[] = {
+		{ "/etc/passwd", -1 },
+		{ "/dev",         0 },
+		{ "/sbin",        0 },
+	};
+
+	for (i = 0; i < NELEMS(arr); i++) {
+		if (tree(arr[i].path, 1) != arr[i].fail)
+			result++;
+	}
+
+	return result;
+}