fix ut error of test_recognize_digits, test=develop

[qili93]

PR types

Others

PR changes

Others

Describe

问题描述：
test_recognize_digits单测在infer时会随机抛出模型参数文件的broken错误，错误log如下：

2020-10-05 17:42:19 InvalidArgumentError: The number of variables to be loaded is 0, expect it to be greater than 0.
2020-10-05 17:42:19   [Hint: Expected out_var_names.size() > 0UL, but received out_var_names.size():0 <= 0UL:0.] (at /paddle/paddle/fluid/operators/load_combine_op.h:42)
2020-10-05 17:42:19   [operator < load_combine > error]

错误原因分析：
由修复test_train_recognize_digits_mlp和test_train_recognize_digits_convd的PR相关https://github.com/PaddlePaddle/Paddle/pull/27475。
在PR27475中修复之后，paddle_build.sh脚本中运行单测的命令如下

export CTEST_PARALLEL_LEVEL=2
ctest -R test_train_recognize_digits_mlp &
ctest -R test_train_recognize_digits_conv &

这两个单测会被并行运行，由于2个都在PR27475中设置依赖test_recognize_digits，test_recognize_digits会同时并行运行2次，log如下：

2020-10-05 17:41:39     ============================================
2020-10-05 17:41:39     Generating TestCases Count ... 
2020-10-05 17:41:39     ============================================
2020-10-05 17:41:39 1 card TestCases count is 425
2020-10-05 17:41:39 Test project /paddle/build
2020-10-05 17:41:39 Test project /paddle/build
2020-10-05 17:41:39         Start 1251: test_recognize_digits
2020-10-05 17:41:39         Start 1251: test_recognize_digits
2020-10-05 17:41:39         Start    2: system_allocator_test
2020-10-05 17:41:39         Start    1: malloc_test

并行运行2个test_recognize_digits时，程序对同一个目录下的同一个param文件进行读写，所以会导致test_recognize_digits中的infer func在load param file时得到的param文件时损坏的。

解决办法：
将test_train_recognize_digits_mlp和test_train_recognize_digits_conv合并为一个单测case，顺序执行就不会有之前的问题。
也尝试过修改cmake文件中set_tests_properties的依赖关系，但是当paddle_build.sh脚本中并行后台运行2个单测时，即使对2个单测互相直接设置了依赖关系，也会导致test_recognize_digits被一起执行2遍，除非某个单测不设置对test_recognize_digits的依赖，这样又会导致test_train_recognize_digits_mlp或者test_train_recognize_digits_conv的单测失败。

[paddle-bot-old]

Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.

[luotao1]

LGTM