unix/coverage: Add coverage test for left adjusted print.

Signed-off-by: Jeff Epler <jepler@gmail.com>

ports/unix/coverage.c

@@ -220,6 +220,7 @@ static mp_obj_t extra_coverage(void) {
         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
+        mp_printf(&mp_plat_print, ".%-3s.\n", "a"); // left adjust
     }
 
     // GC

tests/ports/unix/extra_coverage.py.exp

@@ -14,6 +14,7 @@ false true
 80000000
 abc
 %
+.a  .
 # GC
 0
 0