test-fileio: avoid warning about ineffective comparison

test-fileio: avoid warning about ineffective comparison

On arm64 with gcc-8.2.1-5.fc29.aarch64:
../src/test/test-fileio.c:645:29: warning: comparison is always false due to limited range of data type [-Wtype-limits]
                 assert_se(c == EOF || safe_fgetc(f, &c) == 1);
                             ^~

Casting c to int is not enough, gcc is able to figure out that the original
type was unsigned and still warns. So let's just silence the warning like
in test-sizeof.c.