Clean up SaveChanges/batching tests

[roji]

• SqlServerValueGenerationScenariosTest (functional)
• SqlServerUpdateSqlGeneratorTest (unit tests)
• BatchingTest (functional)
• StoreValueGeneration (functional)
• ReaderModificationCommandBatchTest (unit tests)

(also SQLite, of course)

[AndriySvyryd]

Also move UpdatesTestBase, OptimisticConcurrencyTestBase, TransactionTestBase to .Update
Normalize spelling of 'TPC' and 'TPT' to 'Tpc' and 'Tpt'

[roji]

Also move at least some tests out of ReaderModificationCommandBatchTest (which is a unit test suite) to a functional test suite. Many things being tested there are general behaviors which should get tested across all providers, regardless of whether they use ReaderModificationCommandBatch or not. Probably move DbUpdateException_Entries_are_correct_with_multiple_inserts there too.