unittest-rust: Clean up `asserts` functions

Make a handful of cleanup changes to the `asserts` module's functions.
  * Make `assert_*_inner` private since they're only called by their
    corresponding `assert_*` function.
  * For `assert_err`:
      * Mark `cold` to match `assert_failed`.
      * Take `e` by value since `assert_ok!` stops the test when it
	encounters an `Err` value, so we'll never need `e` to outlive
        the call.
      * Rename to `assert_ok_failed` to match `assert_failed` and to
	avoid confusion if we ever add a `assert_err!` macro.

Bug: None
Test: build
Change-Id: I92414154341fe8e73b9f6b89326a9f9efc58ae8c