test: improve isolation from user configs

## Details

Reported: #667

Sets the `XDG_CONFIG_HOME` environment variable to a non-existant path
at the root of this directory, this should prevent unexpected behavior.

Author: MeanderingProgrammer

doc/render-markdown.txt

@@ -1,5 +1,5 @@
 *render-markdown.txt*                       Improve viewing Markdown in Neovim
-                                  For NVIM v0.12.2    Last change: 2026 May 25
+                                 For NVIM v0.12.3    Last change: 2026 June 18
 
 ==============================================================================
 Table of Contents                          *render-markdown-table-of-contents*

justfile

@@ -25,7 +25,11 @@ bench:
 
 [private]
 busted path:
-  nvim --headless --noplugin -u {{init}} -c "PlenaryBustedDirectory {{path}} {{settings}}"
+  # needed to isolate child processes from user config
+  # as plenary does not pass the --clean flag
+  XDG_CONFIG_HOME="{{justfile_directory()}}/config" \
+    nvim --headless --noplugin -u {{init}} \
+    -c "PlenaryBustedDirectory {{path}} {{settings}}"
 
 health:
   nvim -c "checkhealth render-markdown" -- -