Over the years I have accumulated different unit-tests which use dir() to get all process methods and test them in different circumstances. This produced a lot of code duplication. With this PR I introduce 2 new test classes (process_namespace and system_namespace) which declare all the method names and arguments in a single place, removing a lot cruft and code duplication.

* `Process.wait()` on POSIX now returns an `enum` showing the negative which was used to terminate the process:
```python
>>> import psutil
>>> p = psutil.Process(9891)
>>> p.terminate()
>>> p.wait()
<Negsignal.SIGTERM: -15>
```

* the return value is cached so that the exit code can be retrieved on then next call, mimicking `subprocess.Popen.wait()`
* `Process` object provides more `status` and `exitcode` additional info on `str()` and `repr()`:

```
 >>> proc
 psutil.Process(pid=12739, name='python3', status='terminated', exitcode=<Negsigs.SIGTERM: -15>, started='15:08:20')
```

Extra:
* improved `wait()` doc
* reverted #1736: `psutil.Popen` uses original `subprocess.Popen.wait` method (safer)

Test files/dirs are now removed after each test. when invoked via self.get_testfn().
Until now test files were stored in a global variable and were removed at process exit, via atexit.register(), but this didn't work with parallel tests because the fork()ed workers use os._exit(0), preventing cleanup functions to run.
All test classes now inherit from PsutilTestCase class, which provides the most important methods requiring an automatic cleanup (get_test_subprocess() and others).