lib/libsbuf: add regression tests for sbuf_printf overflow

[souradiptodas6_gmail.com]

Summary

Added test cases to sbuf_string_test.c to validate libsbuf behavior during buffer overflow and error states.

1> sbuf_printf_overflow: Verifies that data is safely truncated and the ENOMEM error code is set when a fixed-length buffer is exceeded.

2> sbuf_printf_test: Validates that sbuf_printf correctly returns -1 when an overflow occurs.

These tests ensure that the string buffer library remains robust against edge cases.

Test Plan

1> Compiled the tests on FreeBSD (CURRENT/14).

2> Ran the tests using atf-run (or via make tests).

3> Verified that all test cases passed and correctly caught the intended error states.

[ngie]

lib/libsbuf/tests/sbuf_string_test.c

296

Please add a newline before this.

[erj]

lib/libsbuf/tests/sbuf_string_test.c

293

The man page says the preferred way to check for an sbuf error is to use the return value from sbuf_finish(); maybe that could be another test case to add.

300

Why not keep this consistent with the test case above and use buf?

[souradiptodas6_gmail.com]

Updated the diff to include the sbuf_finish test case, corrected variable naming for consistency, and added the newline on the desired place. Thank you for the guidance and suggestions . Ready for further review.