[LTP] [RFC PATCH 1/1] doc: Split test-writing-guidelines

[Li Wang]

> > https://github.com/pevik/ltp/wiki/Test-Writing-Guidelines
> > https://github.com/pevik/ltp/wiki/C-Test-API

Seems we have a similar guideline "c-test-tutorial-simple.txt",
maybe better to combine them together?

> > https://github.com/pevik/ltp/wiki/Shell-Test-API

Forgot to mention, with the increased number of docs, do you think it
necessary to create an index for including all documents in a sort?

-- 
Regards,
Li Wang