ci: set test threads to 1 for native provider tests

.github/workflows/ci.yml

@@ -38,7 +38,8 @@ jobs:
         run: cargo build
 
       - name: Tests
-        run: cargo test
+        # there should be a unique test thread for native provider tests (asserting spawned processes count)
+        run: cargo test -- --test-threads 1 
 
   coverage:
     name: Zombienet SDK - coverage
@@ -60,7 +61,8 @@ jobs:
         uses: taiki-e/install-action@cargo-llvm-cov
 
       - name: Collect coverage data
-        run: cargo llvm-cov nextest --lcov --output-path lcov.info
+        # there should be a unique test thread for native provider tests (asserting spawned processes count)
+        run: cargo llvm-cov nextest -j 1 --lcov --output-path lcov.info
 
       - name: Report code coverage
         uses: Nef10/lcov-reporter-action@v0.4.0