testUpgradeInstanceWithOldDataFs: change connection string in Data.fs
This test loads a reference Data.fs, but we don't want to connect to the mysql referenced in the mariadb, because it might be used by another test node.
Before the beginning of the test, rewrite the connection string to the one this test node is using.
This relies on an updated dump produced in https://lab.nexedi.com/nexedi/erp5-bin/commit/8310e4e44c7dcfaecff381e57eda27c64b353d96