fix: in tests, use es6 url from settings

[aaxelb]

Purpose

allow running tests that use es6 locally (...without the effort of addressing various deeper problems here)

Changes

• put ELASTIC6_URL environment variable into its own website-setting
• in conftest.py, use that setting instead of hardcoded localhost url

QA Notes

Please make verification statements inspired by your code and what your code touches.

• Verify
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Any concerns/considerations/questions that development raised?

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