testing/nettest: add checksum zero-length regression test

[imeghar2408-max]

Summary

Add a CMocka regression test for the zero-length fragment handling issue in
checksum() reported in Apache NuttX issue #20010.

When checksum() is called with len == 0 while an odd byte is pending, the
previous implementation could read data[0] even though the current fragment
contained no valid bytes. It could also clear the pending odd-byte state.

The regression test covers this through the public chksum_iob() path using:

AA BB CC | empty fragment | DD EE

and verifies that the chained checksum matches the checksum of the equivalent
contiguous data:

AA BB CC DD EE

This verifies that an empty fragment does not affect the checksum and that
the pending odd-byte state is preserved across the empty fragment.

Changes

• Add test_others_chksum() regression test.
• Register the test in testing/nettest/others/test_others.c.
• Add its declaration to testing/nettest/others/test_others.h.
• Add the test source to testing/nettest/Makefile.
• Add the test source to testing/nettest/CMakeLists.txt.

Impact

This change only adds a regression test to nuttx-apps.
It does not change runtime behavior or production code.

Validation

Regression test with the buggy implementation

The regression test was verified against the implementation without the
zero-length guard and failed as expected:

[ RUN      ] test_others_chksum
[ ERROR    ] --- 30617 != 30396
[ LINE     ] --- others/test_others_chksum.c:52
[  FAILED  ] test_others_chksum
Regression test with the fix applied

With the len == 0 handling fix restored, the test passed:

nsh: mount: mount failed: 20
nsh> cmocka_net_others
[==========] others_tests: Running 2 test(s).
[ RUN      ] test_others_bufpool
[       OK ] test_others_bufpool
[ RUN      ] test_others_chksum
[       OK ] test_others_chksum
[==========] others_tests: 2 test(s) run.
[  PASSED  ] 2 test(s).

The mount failed: 20 message occurs during simulator startup and did not
prevent cmocka_net_others from running successfully.

Checkpatch

The corresponding NuttX fix was checked with the CI-style command:

Used config files:

    1: .codespellrc
✔️ All checks pass.