Revert to run distributed unittest serially, test=develop (#21924)

python/paddle/fluid/tests/unittests/CMakeLists.txt

@@ -278,7 +278,7 @@ if(WITH_DISTRIBUTE)
         set(dist_ut_port 1000)
         foreach(TEST_OP ${DIST_TEST_OPS})
             message(STATUS "set dist_ut_port=${dist_ut_port} on ${TEST_OP}")
-            bash_test_modules(${TEST_OP} MODULES dist_test.sh LABELS "RUN_TYPE=EXCLUSIVE" ENVS "PADDLE_DIST_UT_PORT=${dist_ut_port}")
+            bash_test_modules(${TEST_OP} MODULES dist_test.sh SERIAL LABELS "RUN_TYPE=EXCLUSIVE" ENVS "PADDLE_DIST_UT_PORT=${dist_ut_port}")
             MATH(EXPR dist_ut_port "${dist_ut_port}+50")
         endforeach(TEST_OP)
     endif(NOT APPLE)