[Test] Add unittest_nntrainer_models

[Test] Add unittest_nntrainer_models

This patch generates `NodeWatcher` and `GraphWatcher` to make a test for
unittest_nntrainer_model test.

**Changes proposed in this PR:**
- Add multi iteration test
- Add gtest scafolding

**Additional Patch will be followed**
- Handle loss + activation merge scenario
- Add param test
- Add mnist as a test
- Add final inference result

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>