Problem:    Number of status line items is limited to 80.
Solution:   Dynamically allocate the arrays. (Rom Grk, closes vim/vim#7181)
https://github.com/vim/vim/commit/8133cc6bf454eb90bb0868f7cf806fce5c0c9fe6

The members of stl_item_T have not been prefixed with stl_ contrary to
the vim patch because the amount of stl_ prefixes on single lines of
code in that region was hurtful to readability.

Improve coverage of `build_stl_str_hl`.
Minor removal of dead code in the tested function.

Used

    sed -r -i -e '/ helpers =/ s/$/\nlocal itp = helpers.gen_itp(it)/; s/^(\s*)it\(/\1itp(/' test/unit/**/*_spec.lua

to alter all tests. Locally they all run fine now.

Reasoning:

1. General: state from one test should not affect other tests.
2. Local: travis build is failing with something which may be an output of
   garbage collector. This should prevent state of the garbage collector from
   interferring as well.

also allow handle==0 meaning curbuf/curwin/curtab

This new functionality is explained in the documentation.

Also, many tests have been added to the buffer_spec.lua file

Co-authored-by: Wayne Rowcliffe (@war1025)

Remove helpers.vim_init and simply perform the required initialization in
helpers.lua.