tests/expansion: Use rm instead of unlink

unlink(1) is apparently not always installed everywhere.

Since there is no real difference, just use rm.

tests/expansion.in

@@ -130,7 +130,7 @@ set expandedtilde (env HOME=$tmpdir/linkhome ../test/root/bin/fish -c 'echo ~')
 if test $expandedtilde != $tmpdir/linkhome
 	echo '~ expands to' $expandedtilde ' - expected ' $tmpdir/linkhome
 end
-unlink $tmpdir/linkhome
+rm $tmpdir/linkhome
 rmdir $tmpdir/realhome
 rmdir $tmpdir