[CI] Pass 'TEST_BRANCH' var in trigger-chart-test

What does this MR do?

[CI] Pass 'TEST_BRANCH' var in trigger-chart-test

Passes 'TEST_BRANCH="true"' variable in the 'trigger-chart-test' job to address changes added in gitlab-org/charts/gitlab!3277 (merged) that ensure feature branch pipelines only run when this variable is specified.

Without this fix, the 'trigger-chart-test' trigger job says:

failed - (downstream pipeline can not be created, Pipeline filtered out by workflow rules)

Test plan

Confirm that the job is created from the pipeline view:

To further validate, I triggered that job which created this downstream pipeline.

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated
• New dependencies are managed with dependencies.io