make testing on local servers hermetic

I noticed while fixing issue #2702 that the fish program being tested
was sourcing config.fish files outside of the current build. This also
happens when Travis CI runs the tests but isn't an issue there because
of how Travis is configured to execute the tests.

I also noticed that running `make test` was polluting my personal fish
history; which will become a bigger problem if and when the fishd universal
var file is moved from $XDG_CONFIG_HOME to $XDG_DATA_HOME.

This change makes it possible for an individual to run the tests on
their local machine secure in the knowledge that only the config.fish and
related files from their git repository will be used and doing so won't
pollute their personal fish history.

Resolves #469

tests/interactive.fish

@@ -1,6 +1,8 @@
-#!/usr/local/bin/fish
-#
 # Interactive tests using `expect`
+#
+# There is no shebang line because you shouldn't be running this by hand. You
+# should be running it via `make test` to ensure the environment is properly
+# setup.
 
 # Change to directory containing this script
 cd (dirname (status -f))
@@ -13,6 +15,7 @@ else
 end
 
 source test_util.fish (status -f) $argv; or exit
+cat interactive.config >> $XDG_CONFIG_HOME/fish/config.fish
 
 say -o cyan "Testing interactive functionality"
 if not type -q expect
@@ -21,17 +24,9 @@ if not type -q expect
 end
 
 function test_file
-    rm -Rf tmp.interactive.config; or die "Couldn't remove tmp.interactive.config"
-    mkdir -p tmp.interactive.config/fish; or die "Couldn't create tmp.interactive.config/fish"
-    cat $XDG_CONFIG_HOME/fish/config.fish interactive.config > tmp.interactive.config/fish/config.fish
-    or die "Couldn't create tmp.interactive.config/fish/config.fish"
-
     set -l file $argv[1]
-
     echo -n "Testing file $file ... "
-
     begin
-        set -lx XDG_CONFIG_HOME $PWD/tmp.interactive.config
         set -lx TERM dumb
         expect -n -c 'source interactive.expect.rc' -f $file >$file.tmp.out ^$file.tmp.err
     end