tests/basics: Add test for string module formatting with int argument.

tests/basics/string_format_modulo_int.py

+# test string modulo formatting with int values
+
+# test + option with various amount of padding
+for pad in ('', ' ', '0'):
+    for n in (1, 2, 3):
+        for val in (-1, 0, 1):
+            print(('%+' + pad + str(n) + 'd') % val)