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/resources/testmarkdown/link_syntax.md

@@ -0,0 +1,7 @@
+[index](index)
+[/index](/index)
+[///tmp/some_page](///tmp/some_page)
+[//~/testmarkdown/index](//~/testmarkdown/index)
+[diary:2020-07-22](diary:2020-07-22)
+[link_syntax/nested](link_syntax/nested)
+[./link_syntax/nested](./link_syntax/nested)

test/resources/testmarkdown/link_syntax/nested.md

@@ -0,0 +1,6 @@
+[nested](nested)
+[/index](/index)
+[///tmp/some_page](///tmp/some_page)
+[//~/testmarkdown/index](//~/testmarkdown/index)
+[diary:2020-07-22](diary:2020-07-22)
+[../link_syntax](../link_syntax)

test/resources/testwiki/link_syntax.wiki

@@ -0,0 +1,7 @@
+[[index]]
+[[/index]]
+[[///tmp/some_page]]
+[[//~/testwiki/index]]
+[[diary:2020-07-22]]
+[[link_syntax/nested]]
+[[./link_syntax/nested]]

test/resources/testwiki/link_syntax/nested.wiki

@@ -0,0 +1,6 @@
+[[nested]]
+[[/index]]
+[[///tmp/some_page]]
+[[//~/testwiki/index]]
+[[diary:2020-07-22]]
+[[../link_syntax]]