test/datarepo_src: scope error

test/datarepo_src: scope error

The bus_callback was using a global variable, loop,
while its registerers (e.g., TEST (datareposrc, fps30ReadFlexibleTensors) )
has loop local variable.

Therefore, the callback has been using uninitialied and unrelated
global variable, loop, which often causes unit test errors:

```
[  316s] [ RUN      ] datareposrc.fps30ReadFlexibleTensors
[  317s] Elapsed time: 0.687305 second
[  317s] ../tests/nnstreamer_datarepo/unittest_datareposrc.cc:686: Failure
[  317s] Expected: (0.8) < (elapsed_time), actual: 0.8 vs 0.687305
[  317s] Elapsed time: 0.003219 second
[  318s] Elapsed time: 1.001501 second
[  318s] [  FAILED  ] datareposrc.fps30ReadFlexibleTensors (2045 ms)
```

by exiting from g_main_loop too early.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>