tests/unix/extra_coverage: Add test for mp_vprintf with bad fmt spec.

tests/unix/extra_coverage.py.exp

@@ -11,6 +11,7 @@ false true
 2147483648
 80000000
 80000000
+abc
 # vstr
 tests
 sts

unix/coverage.c

@@ -154,6 +154,7 @@ STATIC mp_obj_t extra_coverage(void) {
         mp_printf(&mp_plat_print, "%u\n", 0x80000000); // should print unsigned
         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
     }
 
     // vstr