Make unnameable tests an error

[djrenren]

Right now the compiler will simply allow tests that cannot be run. The following will compile but will not run any tests.

fn foo() {
  #[test]
  fn bar(){}
}

This constitutes a silent failure and should probably be a hard error (or at least a warning). There's a lint for it: UnnameableTestFunctions but it isn't run by default.

[estebank]

In nightly, introduced in #51450:

warning: cannot test inner function
 --> src/main.rs:2:3
  |
2 |   #[test]
  |   ^^^^^^^
  |
  = note: #[warn(unnameable_test_functions)] on by default

It warns by default, it just hasn't landed in stable yet.