run tracing integration tests single threaded (#1023)

This is a temporary stopgap while we investigate some flaky integration
tests on CI. There is something up with running docker in docker on GH
runners that leads to occasional network connection issues that causes
tests that call the datadog test agent to be flaky.

Author: ekump

.github/workflows/test.yml

@@ -60,10 +60,10 @@ jobs:
         run: cargo test --workspace --exclude builder --doc --verbose && cargo nextest run --workspace --exclude builder --profile ci --verbose -E '!test(tracing_integration_tests::)'
         env:
           RUST_BACKTRACE: 1
-      - name: "[${{ steps.rust-version.outputs.version}}] Tracing integration tests: cargo nextest run --workspace --exclude builder --profile ci --verbose -E 'test(tracing_integration_tests::)'"
+      - name: "[${{ steps.rust-version.outputs.version}}] Tracing integration tests: cargo nextest run --workspace --exclude builder --profile ci --test-threads=1 --verbose -E 'test(tracing_integration_tests::)'"
         if: runner.os == 'Linux'
         shell: bash
-        run: cargo nextest run --workspace --exclude builder --profile ci --verbose -E 'test(tracing_integration_tests::)'
+        run: cargo nextest run --workspace --exclude builder --profile ci --test-threads=1 --verbose -E 'test(tracing_integration_tests::)'
         env:
           RUST_BACKTRACE: 1
       - name: "[${{ steps.rust-version.outputs.version}}] RUSTFLAGS=\"-C prefer-dynamic\" cargo nextest run --package test_spawn_from_lib --features prefer-dynamic -E '!test(tracing_integration_tests::)'"