make the plan wait properly for interactive start

[kn-ms]

moved the helper to a place from where it can be used in the executable as well as in the testcases.

Bug / Requirement Description

In one of the testcase we do not wait for the the proper
interactive startup which cause raceconditions

Solution description

Describe your code changes in detail for reviewers.

use the same helper as elsewhere,
moved the helper to it's own module so
it can be used in test as well in the executable

Checklist:

• Test
• Example (both test_plan.py and .rst)
• Documentation (API)
• News fragment present for release notes
• MS info leakage check
• For new driver: driver index page
• For new assertion: ui/pdf/std renderers, documentation
• For new cmdline arg: documentation