Our tests interfere with each other and cause random failure.

We should improve our test isolation before raising the
level of parallelism.

paddle/scripts/paddle_docker_build.sh

@@ -32,7 +32,7 @@ function start_build_docker() {
     DOCKER_ENV=$(cat <<EOL
         -e FLAGS_fraction_of_gpu_memory_to_use=0.15 \
         -e CTEST_OUTPUT_ON_FAILURE=1 \
-        -e CTEST_PARALLEL_LEVEL=5 \
+        -e CTEST_PARALLEL_LEVEL=1 \
         -e APT_MIRROR=${apt_mirror} \
         -e WITH_GPU=ON \
         -e CUDA_ARCH_NAME=Auto \