Fix driver tests flakiness (#3185)

# Description
It seems like running the driver tests in a single-thread mode fixes the
issue with accidentally stuck
tests(https://github.com/cowprotocol/services/actions/runs/11141574437/job/30962723813).
During a couple of runs using the flaky runner job, not a single driver
test failed. The only disadvantage is that these tests execute now 5m
instead of 2m.

.github/workflows/pull-request.yaml

@@ -167,7 +167,7 @@ jobs:
       # Build the driver's tests.
       - run: cargo build -p driver --tests
       # Don't spawn any docker containers. The driver's tests will spawn anvil itself.
-      - run: cargo nextest run -p driver --run-ignored ignored-only
+      - run: cargo nextest run -p driver --test-threads 1 --run-ignored ignored-only
 
   openapi:
     timeout-minutes: 60