Add unit test for fexist()

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

tests/fexist.c

@@ -0,0 +1,23 @@
+#include "check.h"
+
+int main(void)
+{
+	size_t i = 0;
+	struct { char *file; int exist; } arr[] = {
+		{ "/etc/passwd", 1 },
+		{ "/etc/kalle",  0 },
+		{ "/sbin/init",  1 },
+		{ "/dev/null",   1 },
+		{ NULL,  0 },
+	};
+
+	for (i = 0; i < NELEMS(arr); i++) {
+		if (fexist(arr[i].file) != arr[i].exist)
+			err(1, "Failed fexist(%s)", arr[i].file ?: "NULL");
+		else
+			printf("File %-11s %-14s => OK!\n", arr[i].file ?: "NULL",
+			       arr[i].exist ? "does exist" : "does not exist");
+	}
+
+	return 0;
+}