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>

doc/vimwiki.txt

@@ -1054,13 +1054,17 @@ absolute to the wiki root directory, that is, the link [[/index]] always opens
 the file /path/to/your/wiki/index.wiki, no matter in which subdirectory you
 are currently in.
 
-If you want an absolute path in your local box you can prefix the path
-with // >
+If you want to use an absolute path to a wiki page on your local filesystem,
+you can prefix the path with // >
   [[//absolute_path]]
 For example: >
   [[///tmp/in_root_tmp]]
   [[//~/in_home_dir]]
   [[//$HOME/in_home_dir]]
+In a wiki with the default wiki extension, this link: >
+  [[///tmp/foo]]
+Links to the file: >
+  /tmp/foo.wiki
 
 Links to subdirectories inside the wiki directory are also supported. They
 end with a "/": >
@@ -3894,6 +3898,7 @@ Contributors and their Github usernames in roughly chronological order:
     - Yifan Hu (@yhu266)
     - Levi Rizki Saputra (@levirs565)
     - Fergus Collins (@C-Fergus)
+    - Brennen Bearnes
 
 ==============================================================================
 16. Changelog                                              *vimwiki-changelog*
@@ -3955,6 +3960,7 @@ Changed:~
 Removed:~
 
 Fixed:~
+    * PR #1108: Fix resolution of leading-slash page links, add link tests
     * Allow title values with quotes
     * Enable strikethrough for Neovim
     * Issue #1029: Fix: error loading plugin when lang uses comma instead of