Use only one test thread

Otherwise the builds made by `TestEnv::command` will conflict with each
other. This manifested itself on Windows with

> linking with `link.exe` failed: exit code: 1104
> LINK : fatal error LNK1104: cannot open file '...'

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
       - name: Build with logging and webpki roots
         run: cargo build --features logging,webpki-roots --no-default-features
       - name: Run tests
-        run: cargo test
+        run: cargo test -- --test-threads 1
 
   clippy:
     name: run clippy lints