Test modules should always be last

```
error: items after a test module
   --> build/make/tools/aconfig/aconfig/src/test.rs:18:1
    |
18  | pub mod test_utils {
    | ^^^^^^^^^^^^^^^^^^
...
345 | pub use test_utils::*;
    |         ^^^^^^^^^^
    |
    = help: for further information visit
https://rust-lang.github.io/rust-clippy/master/index.html#items_after_test_module
    = note: `-D clippy::items-after-test-module` implied by `-D
warnings`
    = help: to override `-D warnings` add
`#[allow(clippy::items_after_test_module)]`
```

Bug: http://b/321303117
Test: toolchain/android_rust/test_compiler.py --prebuilt-path dist/rust-dev.tar.xz --target aosp_cf_x86_64_phone --all-rust
Change-Id: I2c95af50a5d389fc37e1abc677e9b4d34b6562b4