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/expansion.in

@@ -94,7 +94,7 @@ set tmpdir $PWD
 cd $saved
 mkdir $tmpdir/realhome
 ln -s $tmpdir/realhome $tmpdir/linkhome
-set expandedtilde (env HOME=$tmpdir/linkhome ../fish -c 'echo ~')
+set expandedtilde (env HOME=$tmpdir/linkhome ../test/root/bin/fish -c 'echo ~')
 if test $expandedtilde != $tmpdir/realhome
 	echo '~ expands to' $expandedtilde ' - expected ' $tmpdir/realhome
 end