test-alias: add some tests to ensure we aren't double-substituting

An earlier iteration of an upcoming patch caused inadvertent
double substitution. Ensure we have test coverage for this.

tests/test-alias.t

@@ -229,6 +229,10 b' simple shell aliases'
   foo
   $ hg echoall 'test $2' foo
   test $2 foo
+  $ hg echoall 'test $@' foo '$@'
+  test $@ foo $@
+  $ hg echoall 'test "$@"' foo '"$@"'
+  test "$@" foo "$@"
   $ hg echo1 foo bar baz
   foo
   $ hg echo2 foo bar baz