Change CI tests to run sequentially

.github/workflows/main.yml

@@ -24,7 +24,7 @@ jobs:
         name: cargo test
         with:
           command: test
-          args: --all-targets --benches
+          args: --all-targets --benches -- --test-threads=1
       - uses: actions-rs/cargo@v1
         name: cargo fmt
         with:

CONTRIBUTING.md

@@ -46,3 +46,8 @@ cargo clippy
     2. Error String: Lower case, no `.` terminating.
     3. Display imps: Lower case, no `.` terminating.
 
+### Integration tests
+
+Note that the way the integration tests run now, it's best to run them sequentially rather than concurrently (which is Rust's default). You can run them sequentially with this command:
+
+`cargo test -- --test-threads=1`