Fix for control flow behavior regression

[tilln]

Description

Programmatic manipulation of the control flow via API methods of JMeterContext is not working as it used to.

When JMeter v5.0 introduced the "ability to Switch to next iteration of Current Loop" (see Bug 62238) the changes to JMeterThread were not entirely logically equivalent and broke some of our existing test when migrating to v5.x.

The simplest test plan to demonstrate this is as follows, where the Debug Sampler should never be run.
(This works up to JMeter v4.0 and fails in v5.0 and above.)

Test Plan
    \ Thread Group
        \ JSR223 Sampler
            ctx.setStartNextThreadLoop(true)
        \ Debug Sampler

Reason is the following change (refer here)

When processing the sample result, the "logical action" would only be changed if the result carried the "restart flag":

if(result.isStartNextThreadLoop()) {	            
    threadContext.setStartNextThreadLoop(true);	
}

But now the result unconditionally overwrites the context's "logical action":

threadContext.setTestLogicalAction(result.getTestLogicalAction());

This PR proposes to make the action change conditional again: only if different than CONTINUE

if (result.getTestLogicalAction() != TestLogicalAction.CONTINUE) {
    threadContext.setTestLogicalAction(result.getTestLogicalAction());
}

Motivation and Context

This fixes a breaking change from JMeter v4.0 to v5.0 that was supposed to be non-breaking.
Workarounds involve changing existing test plans by for example using
prev.setStartNextThreadLoop(true) (or the non-deprecated equivalent)
but this does not work everywhere e.g. in Preprocessors where there is no SampleResult yet.

How Has This Been Tested?

Has not yet been tested due to the trivial nature of the change.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the [code style][style-guide] of this project.
• I have updated the documentation accordingly.

[vlsi]

Could you please add a test to validate the change?