(TST) Replacing Timestamp.now () in tests.
- [x] xref #44341
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry

Replacing Timestamp.now() in tests.
Left Timestamp.now () in the following tests:

```
scalar/timestamp/test_timestamp.py:290:        compare(Timestamp.now(), datetime.now())
scalar/timestamp/test_timestamp.py:291:        compare(Timestamp.now("UTC"), datetime.now(timezone("UTC")))
scalar/timestamp/test_timestamp.py:320:        compare(Timestamp.now(), datetime.now())
scalar/timestamp/test_timestamp.py:321:        compare(Timestamp.now("UTC"), datetime.now(tzutc()))
scalar/timestamp/test_constructors.py:480:        ts_from_method = Timestamp.now()
scalar/timestamp/test_constructors.py:484:        ts_from_method_tz = Timestamp.now(tz="US/Eastern")
```
  
