This helpers does the following:
1. starts a server
2. writes a script/config
3. verifies invariants
4. writes a new script/config
5. reloads
6. verifies invariants after reload

This patch allows to set not only script, but config too on the step 4,
before the reload.

Part of #8967

NO_DOC=test helper upgrade
NO_CHANGELOG=see NO_DOC
NO_TEST=see NO_DOC