Properly use test_fetcher for examples
# What does this PR do?

This PR fixes the `test_fetcher` util script for examples. It does so by adding the example test file whenever an example file is modified, or if said example file is modified (anything in examples/pytorch).
To avoid this being launched by the other test jobs, the default for the filters arguments is changed to "tests".

An example modification will now run the example test file: on the commit "Fake example modification", the test fetcher for the example script returns:
```
Master is at 421929b556aedf022a1c4a1f3b2f116b14a7b88a
Current head is at 5ba342cbf98664f5a7a98d6f02b798d78ee523f3
Branching commit: 421929b556aedf022a1c4a1f3b2f116b14a7b88a

### DIFF ###


### MODIFIED FILES ###
- examples/pytorch/language-modeling/run_clm.py
- utils/tests_fetcher.py

### IMPACTED FILES ###
- examples/pytorch/language-modeling/run_clm.py
- utils/tests_fetcher.py

### TEST TO RUN ###
- examples/pytorch/test_examples.py
```

In practice the examples job is run when any modification triggers any tests (so it's run when there is a real modification in the examples or the source code of transformers). Before this PR, the example modifications were ignored.