clustering test was heavy

Fix [jubatus:issue#739](https://github.com/jubatus/jubatus/issues/739)

default `core::clustering` parameter is big enough to BigData.
But that parameter is too heavy for test in small dataset.
So we should set small parameter for `clustering` and use small dataset for rapid test running.

## Before

```
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/0
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/0 (1600 ms)
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/1
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/1 (1596 ms)
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/2
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/2 (5682 ms)
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/3
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/3 (5724 ms)
```

## After

```
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/0
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/0 (66 ms)
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/1
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/1 (65 ms)
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/2
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/2 (235 ms)
[ RUN      ] clustering_test_instance/clustering_test.get_nearest_members/3
[       OK ] clustering_test_instance/clustering_test.get_nearest_members/3 (241 ms)
```