This commit fixes a bug in no-split and allow-split ring buffers free buffer size calculation.
When the free size available in the buffers less than the size of one item header,
the function prvGetCurMaxSizeNoSplit/AllowSplit() incorrectly returned the maxItemSize instead of 0.
This is due to the comparision between a negative and a positive value
where both operands are treated as unsigned during the comparision operation,
thereby treating the negative operand as a large integer.
Also added new unit tests to test buffer-full and almost-full conditions
where this scenario is likely to be hit.
Closes https://github.com/espressif/esp-idf/issues/7344
Closes https://github.com/espressif/esp-idf/pull/7371
7c0d99e68d