rust: run Clippy on tests and fix existing errors

Summary:
We run Clippy to catch common errors, but by default Clippy only runs on
non-test code. We noticed this in #4487, when Clippy would have caught a
real error (test doing nothing due to `unit_cmp`). This patch adds the
`--tests` flag to our Clippy invocation and fixes all existing warnings.

Test Plan:
The first draft of this PR will include only the workflow change, and so
should fail…

wchargin-branch: rust-clippy-tests
wchargin-source: 3be9bc4b9c2084a9f9dda2b0082227f061989811

Author: wchargin

.github/workflows/ci.yml

@@ -243,7 +243,7 @@ jobs:
         uses: actions-rs/clippy-check@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
-          args: --manifest-path tensorboard/data/server/Cargo.toml
+          args: --tests --manifest-path tensorboard/data/server/Cargo.toml
       - name: 'Check cargo-raze freshness'
         run: |
           rm -rf third_party/rust/