test/compile-fail: clarify bad-for-loop

There are two distinct kinds of bad-for-loops to check for with two
distinct error messages.  The current bad-for-loop.rs does not make this
clear, so rewrite it into bad-for-loop{,-2}.rs.  The two failing
prototypes are:

  fn quux(_: &fn(&int) -> int) -> bool { true }
  fn quux(_: &fn(&int) -> bool) -> () { }

Note that the second one runs fine in rust-0.6 (the latest release), and
only fails with a rust built from `incoming`.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
tutorial: fix for-loop example

Although in the example function `each` works as expected with
rust-0.6 (the latest release), it fails to even compile with `incoming`
rust (see test/compile-fail/bad-for-loop-2.rs). Change the function to
return a `bool` instead of `()`: this works fine with both versions of
rust, and does not misguide potential contributors.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
auto merge of #6972 : artagnon/rust/bad-for-loop, r=bstrie

I noticed this while reading the tutorial.

[1/2] adds tests to guard against regressions.
[2/2] corrects the tutorial.

Please let me know if you have a standard place to put tests: they seem to be all over the place currently.