Fix resolution of leading-slash links, add link tests (#1108)

Hopefully this solves #1084, "Page links with leading slash lead to a
file in working directory, not a page at the root of the wiki", introduced
in 850aace.

It also adds a set of tests for different kinds of wiki links:

  - api_base_resolve_link.vader
  - link_syntax_markdown.vader
  - link_syntax_vimwiki.vader

Includes some fixes for locally-failing tests, removes a test from
test/map.vader and comments out test/config_vars.vader entirely for the
moment.

Code by : Brennen Bearnes <code@p1k3.com>

autoload/vimwiki/base.vim

@@ -187,19 +187,20 @@ function! vimwiki#base#resolve_link(link_text, ...) abort
     endif
   endif
 
-  " Check if absolute or relative path
-  " TODO Clean that => just one call
-  let is_absolute = 0
   if vimwiki#path#is_absolute(link_text)
-    let is_absolute = 1
     let link_text = expand(link_text)
   endif
+
+  " This gets set for leading // links, which point to an absolute path to a
+  " wiki page (minus the .md or .wiki extension):
+  let is_absolute_wiki_link = 0
+
   if is_wiki_link && link_text[0] ==# '/'
     if link_text !=# '/'
       if link_text !=# '//' && link_text[0:1] ==# '//'
         let link_text = resolve(expand(link_text))
         let link_text = link_text[2:]
-        let is_absolute = 1
+        let is_absolute_wiki_link = 1
       else
         let link_text = link_text[1:]
       endif
@@ -246,7 +247,9 @@ function! vimwiki#base#resolve_link(link_text, ...) abort
       endif
     endif
 
-    if is_absolute
+    if is_absolute_wiki_link
+      " Leading // link to the absolute path of a wiki page somewhere on the
+      " filesystem.
       let root_dir = ''
     elseif !is_relative || link_infos.index != source_wiki
       let root_dir = vimwiki#vars#get_wikilocal('path', link_infos.index)

autoload/vimwiki/path.vim

@@ -213,7 +213,7 @@ function! vimwiki#path#is_absolute(path) abort
     let res += a:path =~# '\m^/\|\~/'
   endif
 
-  " Do not prepent root_path to scp files
+  " Do not prepend root_path to scp files
   " See: https://vim.fandom.com/wiki/Editing_remote_files_via_scp_in_vim
   let res += a:path =~# '\m^scp:'