Renamed prepareIdMappings() in MigrateTestBase

Problem/Motivation

Over in #2314289: Track result of migrations and use it to properly define migrate_dependencies we're now tracking the migration result in the database. This means that we have to manually set the migration result on dependant migrations during test setup. We added $migration->setMigrationStatus() to prepareIdMappings() with the vision of renaming it to something more appropriate. That's what we're going to do here.

Proposed resolution

Decide on a name and then find/replace all occurrences.

Remaining tasks

Wait for #2314289: Track result of migrations and use it to properly define migrate_dependencies and then roll a patch.

User interface changes

n/a

API changes

MigrationTestBase::prepareIdMappings() will be renamed.