Add regression coverage for integer conversions with explicit bases from 2 through 36. Exercise lower- and upper-case digits, positional values, all six strto* interfaces, end pointers, errno, and the invalid range boundaries.
The tests fail at base 27 without apache/nuttx#19594 and pass with the corresponding libc fix.
Signed-off-by: hanzhijian <hanzhijian@zepp.com>
