Edit of TestRun CRD during the test

[yorugac]

Currently editing of TestRun while the test is running or even after it has finished is not supported. To be precise, one can edit it if they want but they shouldn't as that
a) will likely not result in the expected change of behaviour if it happens while the test is running;
b) may lead to failures if it happens while the test is running;
c) won't do anything if it happens after the test run has finished.

While the test run is running
Executing apply of yaml with the same name and namespace but different options in the middle of the test run will result in message testrun.k6.io/k6-sample configured and then the controller's logic can become broken. E.g. if one increases parallelism, then the controller is stuck waiting. If parallelism decreases, it might "survive" in OSS usage but not change number of runners. In Cloud usage, it may lead to additional failures.

Similarly, modification of the CR during test run execution will result in message testrun.k6.io/k6-sample edited and the same issues as above.

When the test run has finished
Edits of CRD won't have any impact at all.

Opening this issue as a follow-up from #138. Feedback and thoughts are welcome.

[yorugac]

Updated the issue description to take into account the use case described in #312