Remove extraneous mod statements from tests

[caass]

Description

Integration tests are compiled into their own binaries, which are then
executed. As such, there's no need to wrap integration tests in modules:

// in `tests/some_test.rs`
use some::dependency;

#[cfg(test)]
mod some_integration_test {
  use super::*;

  // Some tests go here.
}

This is common practice in unit tests, which if not gated behind a
cfg flag are included in the final release binary. We were mistakenly
doing the same thing in integration tests, where it was unnecessary.

This PR just removes all those module declarations.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

bazel test //... passes

Checklist

• Updated documentation if needed
• Tests added/amended
• bazel test //... passes locally
• PR is contained in a single commit, using git amend see some docs

---

This change is 

[allada]

Reviewed 34 of 34 files at r1, all commit messages.
Reviewable status: 1 of 1 LGTMs obtained, and pending CI: Bazel Dev / ubuntu-22.04, Cargo Dev / macos-13, Cargo Dev / ubuntu-22.04, asan / ubuntu-22.04, docker-compose-compiles-nativelink (20.04), docker-compose-compiles-nativelink (22.04), macos-13, pre-commit-checks, ubuntu-20.04 / stable, ubuntu-22.04, ubuntu-22.04 / stable, vale, windows-2022 / stable