tox.ini/stestr: remove "--serial" to enable test parallelization (#880)

Having "--serial" impedes the parallelization and thus affects the
unit test execution speed greatly. We have no real reason to have
"--serial" in place, it both makes the test execution slower and
possibly hides hidden dependencies between the test cases. The unit
tests must be able to run in parallel and should be indifferent to
the ordering.

This patch removes the "--serial" argument, adds "--random" and
"--slowest" arguments, which randomizes the order of the test cases
and reports the slowest test cases respectively.

Below are the run time differences between the baseline(main) and
this patch(stestr-parallel):

Command `tox`:
---------------------------------------------------
main:
```
py3: OK (66.73=setup[0.03]+cmd[0.06,66.64] seconds)
...
congratulations :) (111.38 seconds)
```

stestr-parallel:
```
py3: OK (10.21=setup[0.03]+cmd[0.05,10.12] seconds)
...
congratulations :) (53.62 seconds)

```

results:
6.53x improvement in "py3" runtime
2.07x improvement in total runtime

Command `tox --parallel`:
---------------------------------------------------
main:
```
py3: OK (68.94=setup[0.17]+cmd[0.05,68.71] seconds)
...
congratulations :) (71.23 seconds)

```

stestr-parallel:

```
py3: OK (12.12=setup[0.21]+cmd[0.06,11.86] seconds)
...
congratulations :) (41.61 seconds)
```

results:
5.68x improvement in "py3" runtime
1.71x improvement in total runtime

Signed-off-by: Mustafa Kemal Gilor <[email protected]>

tox.ini

@@ -23,7 +23,7 @@ deps =
     -r{toxinidir}/test-requirements.txt
 commands =
     coverage erase
-    stestr run --serial --test-path {[testenv]unit_tests} {posargs}
+    stestr run --random --slowest --test-path {[testenv]unit_tests} {posargs}
 
 [testenv:pep8]
 allowlist_externals = flake8