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>

test/link_generation.vader

@@ -29,6 +29,8 @@ Expect (The links with a header):
   = Generated Links =
           - [[buzz_bozz]]
           - [[index]]
+          - [[link_syntax]]
+          - [[link_syntax/nested]]
 
 Execute (VimwikiGenerateLinks x 2):
   edit $HOME/testwiki/Test.wiki
@@ -42,6 +44,8 @@ Expect (The links with a header (bis)):
   = Generated Links =
           - [[buzz_bozz]]
           - [[index]]
+          - [[link_syntax]]
+          - [[link_syntax/nested]]
 
   Last Line
 
@@ -75,6 +79,8 @@ Expect (The links with a header):
 
   - [Buzz Bozz](buzz_bozz)
   - [Test Wiki](index)
+  - [link_syntax](link_syntax)
+  - [link_syntax/nested](link_syntax/nested)
 
 Do (Save Test.md && Re-GenerateLinks):
   :edit $HOME/testmarkdown/Test.md\<CR>
@@ -90,6 +96,8 @@ Expect (The links with a header with file Test):
   - [Generated Links](Test)
   - [Buzz Bozz](buzz_bozz)
   - [Test Wiki](index)
+  - [link_syntax](link_syntax)
+  - [link_syntax/nested](link_syntax/nested)
 
 Execute (Clean: Remove Test.md):
   call DeleteFile('$HOME/testmarkdown/Test.md')
@@ -180,6 +188,12 @@ Do (Edit diary.md && GenerateDiaryLinks):
 Expect (diary index generated):
   # Diary
 
+  ## 2020
+
+  ### July
+
+  - [2020-07-22](2020-07-22)
+
   ## 2019
 
   ### December