Use current time as start time for OMTG

[FSchumacher]

Description

Use current time as of actually starting the open model thread group

Motivation and Context

When we use the test start time via JMeterContextService#getTestStartTime, we might create a storm of test samples at the beginning, when a setup thread group made us wait.

Related to #6352

How Has This Been Tested?

Used the test plan attached and looked at the req/s values. They should be in the range of 1 req/s. omtg-and-setup-group.jmx.zip

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• Might be a Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

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