modify StorageTimestampTests::InitialSyncSetIndexMultikeyOnInsert to build indexes in background

In recent weeks, the test StorageTimestampTests::InitialSyncSetIndexMultikeyOnInsert has been failing intermittently with the following invariant message:

[repl-index-builder-1] Invariant failure !mySnapshot || *mySnapshot <= commitTimestamp commit timestamp Timestamp(1, 87) cannot be older than current read timestamp Timestamp(1, 92) src/mongo/db/catalog/index_timestamp_helper.cpp 88

A workaround is proposed in SERVER-39981.

However, we should investigate why this test case does not pass consistently with background/hybrid index builds and determine if there is an underlying issue in the index build machinery.