modify StorageTimestampTests::InitialSyncSetIndexMultikeyOnInsert to build indexes in foreground

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

Since the intent of the test case is not to test background/hybrid builds, it should be fine to have this test build its indexes in the foreground until we get a better understanding why this test is not interacting well with background/hybrid index builds.

A TODO comment in the code should mention SERVER-39982.