test: more thorough system location interference avoidance

Closes #309

test/config/bashrc

@@ -27,11 +27,13 @@ export PATH=$PATH:/usr/games:/usr/local/games
 # For clean test state, avoid sourcing user's ~/.bash_completion
 export BASH_COMPLETION_USER_FILE=/dev/null
 
-# ...and avoid stuff in BASH_COMPLETION_USER_DIR overriding in-tree
-# completions. The user dir is first in the lookup path, so this should also
-# give precedence to the in-tree "completions" dir over other ones, e.g.
-# the one possibly in /usr/share/bash-completion.
+# ...and avoid stuff in BASH_COMPLETION_USER_DIR and system install locations
+# overriding in-tree completions. Setting the user dir would otherwise suffice,
+# but simple xspec completions are only installed if a separate one is not
+# found in any completion dirs. Therefore we also point the "system" dirs to
+# locations that should not yield valid completions and helpers paths either.
 export BASH_COMPLETION_USER_DIR=$(cd "$SRCDIR/.."; pwd)
+export XDG_DATA_DIRS=/var/empty
 
 # Make sure default settings are in effect
 unset -v \