load-tester: Fix load-tester on platforms where plain `char` is signed

load-tester: Fix load-tester on platforms where plain `char` is signed

fgetc() returns an int and EOF is usually -1 so when this gets casted to
a char the result depends on whether `char` means `signed char` or
`unsigned char` (the C standard does not specify it).  If it is unsigned
then its value is 0xff so the comparison with EOF will fail as that is an
implicit signed int.