Fix random test failures

"test_networkbench: increase resolution time tolerance" addresses [this](https://erp5.nexedi.net/test_result_module/20200911-8C18D791/14):

```
======================================================================
FAIL: test_dnsbench_fail (slapos.test.test_networkbench.TestDNSBench)
Test dns failure resolution
----------------------------------------------------------------------
Traceback (most recent call last):
File "/srv/slapgrid/slappart20/srv/testnode/cum/inst/test0-0/parts/slapos.toolbox/slapos/test/test_networkbench.py", line 66, in test_dnsbench_fail
self.assertLess(info[3], 1)
AssertionError: 2.0736539363861084 not less than 1
```

"cpu load plugin: fix test and minor cleanups" addresses [this](https://erp5.nexedi.net/test_result_module/20200911-C444DB47/13):

```
======================================================================
ERROR: test_check_cpu_load_run (slapos.test.promise.plugin.test_check_server_cpu_load.TestCheckServerCPULoad)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/srv/slapgrid/slappart9/srv/testnode/bvi/inst/test0-0/parts/slapos.toolbox/slapos/test/promise/plugin/test_check_server_cpu_load.py", line 48, in test_check_cpu_load_run
    self.launcher.run()
  File "/srv/slapgrid/slappart9/srv/testnode/bvi/soft/65dbbd3d11daa99cf78c9566a0a4798e/parts/slapos.core-repository/slapos/grid/promise/__init__.py", line 884, in run
    failed_promise_output))
PromiseError: Promise 'server-cpu-load-promise.py' failed with output: Error: Promise timed out after 0.5 seconds
```
Some more fixes are in nexedi/slapos!809

See merge request nexedi/slapos.toolbox!85