lib: test_scanf: Add explicit type cast to result initialization in test_number_prefix()

A recent change in clang allows it to consider more expressions as
compile time constants, which causes it to point out an implicit
conversion in the scanf tests:

  lib/test_scanf.c:661:2: warning: implicit conversion from 'int' to 'unsigned char' changes value from -168 to 88 [-Wconstant-conversion]
    661 |         test_number_prefix(unsigned char,       "0xA7", "%2hhx%hhx", 0, 0xa7, 2, check_uchar);
        |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  lib/test_scanf.c:609:29: note: expanded from macro 'test_number_prefix'
    609 |         T result[2] = {~expect[0], ~expect[1]};                                 \
        |                       ~            ^~~~~~~~~~
  1 warning generated.

The result of the bitwise negation is the type of the operand after
going through the integer promotion rules, so this truncation is
expected but harmless, as the initial values in the result array get
overwritten by _test() anyways. Add an explicit cast to the expected
type in test_number_prefix() to silence the warning. There is no
functional change, as all the tests still pass with GCC 13.1.0 and clang
18.0.0.

Cc: stable@vger.kernel.org
Link: https://github.com/ClangBuiltLinux/linuxq/issues/1899
Link: https://github.com/llvm/llvm-project/commit/610ec954e1f81c0e8fcadedcd25afe643f5a094e


Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230807-test_scanf-wconstant-conversion-v2-1-839ca39083e1@kernel.org