chore: Fix make test warning

[u5surf]

• sprintf -> snprintf in test.c

I found build warnings on MacOS. so I fixed with snprintf following warning message.

% make test
cc -Wall -fsanitize=address,undefined  -o test-bin picohttpparser.c picotest/picotest.c test.c
test.c:480:25: warning: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Wdeprecated-declarations]
        bufsz = (size_t)sprintf(buf, "%zx%s\r\n", chunk_len, extra);
                        ^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.0.sdk/usr/include/_stdio.h:274:1: note: 'sprintf' has been explicitly marked deprecated here
__deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
^
/Library/Developer/CommandLineTools/SDKs/MacOSX15.0.sdk/usr/include/sys/cdefs.h:218:48: note: expanded from macro '__deprecated_msg'
        #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))
                                                      ^
1 warning generated.

[kazuho]

Thank you for the PR; merged.