Tests: Silence a warning from GCC 15.1

Tests: Silence a warning from GCC 15.1

It was (probably) intentionally without the null terminator, but the test
works with null terminator too (the test still fails with xz <= 5.0.3),
so simply omit one character to silence the warning.

tests/test_bcj_exact_size.c:30:32: error: initializer-string for array of ‘unsigned char’ truncates NUL terminator but destination lacks ‘nonstring’ attribute (17 chars into 16 available) [-Werror=unterminated-string-initialization]
   30 |         const uint8_t in[16] = "0123456789ABCDEF";
      |                                ^~~~~~~~~~~~~~~~~~

Fixes: d8db706acb83 ("liblzma: Fix possibility of incorrect LZMA_BUF_ERROR.")
Fixes: https://github.com/tukaani-project/xz/issues/176