Add more tests from stringlib

tests/string_strncat.c

@@ -0,0 +1,20 @@
+#include "string.h"
+
+int main(void)
+{
+  char buffer[32];
+  char *src = "Test String";
+  int i = 0 ;
+  if ( strncat((char *)&buffer, NULL, 0) != 0 ) return 1;
+  if ( strncat(NULL, NULL, 0) != 0 ) return 2;
+  if ( strncat(NULL, src, 0) != 0 ) return 3;
+
+  if ( strncat((char *)&buffer, src, 11) == 0 ) return 4;
+
+  for ( i = 0; i < 11 ; i++  ) {
+    if ( buffer[i] != *(src + (sizeof(char) * i)) ) return i + 5; 
+  }
+  if ( buffer[11] != '\0' ) return 12 + 5;
+
+  return 0;
+}