Concurrent test execution

In test conf object add concurrency group:
```
variables:
foo: bar
config:
concurrency:
enabled: true
group: teamA
steps:
....
```
If config.concurrency.enabled is true - test can be run concurrent.
If config.concurrency.group is true - no tests of the same group can be run at once.
Should use multiprocessing and create pool with CPU + 1
Test results in json/html should be concurrent safe.

Start by locating the test configuration object and the code that schedules test execution. Trace how config.concurrency.enabled and config.concurrency.group should control a multiprocessing pool of CPU + 1, then verify that JSON and HTML results remain safe when tests run concurrently and that tests in the same group do not overlap.

Written by the indexing model from the issue text.