add more alias unit tests

A recent regression to the `alias` command points out the need for more
unit tests of its behavior. I also decided to use it as an opportunity
to normalize the output of just `alias` to list aliases.

tests/alias.out

@@ -1,2 +1,5 @@
 ran foo
 foo ran
+# bare `alias` should list the aliases we have created and nothing else
+alias a-2 'echo "hello there"'
+alias my_alias 'foo; and echo foo ran'