Clean up Feature Migration tests

[gwbrown]

There's some cleanup to be done in FeatureMigrationIT and related classes.

• Convert static preMigrationHook and postMigrationHook to use instance members, retrieved from the internalCluster()
• Use Hamcrest matchers as described in this comment for better failure output
• Rename createSimpleSettings to createSettings
• Rename createSimpleMapping to createMapping
• add support for unknown fields [7.17] Register Feature migration persistent task state named XContent (#84192) #84258 (review)

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)