Add integration test file for nas example (#1148)

test/config_test/examples/mnist-nas.test.yml

+authorName: nni
+experimentName: default_test
+maxExecDuration: 5m
+maxTrialNum: 4
+trialConcurrency: 2
+
+tuner:
+  codeDir: ../../../examples/tuners/random_nas_tuner
+  classFileName: random_nas_tuner.py
+  className: RandomNASTuner
+trial:
+  codeDir: ../../../examples/trials/mnist-nas
+  command: python3 mnist.py --batch_num 100
+  gpuNum: 0
+
+useAnnotation: true
+multiPhase: false
+multiThread: false
+
+trainingServicePlatform: local