litValue is a confusing method name in testing code

**Type of issue**: other enhancement

**Impact**: API addition (no impact on existing code)

ChiselTest returns a Chisel type on a ```peek()```. For more interesting tests we need to convert those into Scala land. ```litValue``` returns a ```BigInt```. However, the name is confusing as during testing those returned values are not constants/literals. They might change every clock cycle. The Proposal is to rename this method to ```asBigInt``` (adding it for now and maybe getting rid of ```litValue``` some time).