Cover the recently introduced inline routines which back the preprocessor
macros for endianess conversion. Add test sequences for read and write
routines for different data types of different sizes, different endianess
formats and signedness, and include those routines which increment the
read/write position.
Introduce a new tests/conv.c source file which exercises the endianess
conversion macros. It's assumed that some use cases may break their
operation, fortunately these edge cases were not seen before, or the
unreliable operation went unnoticed. This test raises awareness of the
implementation's constraints.
This is a start, the test sequence will benefit from adding some more
cases to increase coverage.