unix/coverage: Add test for printing literal % character.

ports/unix/coverage.c

@@ -161,6 +161,7 @@ STATIC mp_obj_t extra_coverage(void) {
         mp_printf(&mp_plat_print, "%x\n", 0x80000000); // should print unsigned
         mp_printf(&mp_plat_print, "%X\n", 0x80000000); // should print unsigned
         mp_printf(&mp_plat_print, "abc\n%"); // string ends in middle of format specifier
+        mp_printf(&mp_plat_print, "%%\n"); // literal % character
     }
 
     // GC

tests/unix/extra_coverage.py.exp

@@ -13,6 +13,7 @@ false true
 80000000
 80000000
 abc
+%
 # GC
 0
 0