Fix failing LiveTradingDataFeed unit tests
Fix failing LiveTradingDataFeed unit tests

- In some tests SymbolCache.GetSymbol was being called before AddData, throwing an exception when launched as single tests.
- HandlesManyCustomDataSubscriptions was failing both locally and in Travis (only occasionally). This behavior started since #3040 was merged (LiveSynchronizer is now using a 500ms timeout for the reset event) and the asserts have been updated to reflect this.

jameschch/Lean,JKarathiya/Lean,QuantConnect/Lean,StefanoRaggi/Lean,JKarathiya/Lean,jameschch/Lean,StefanoRaggi/Lean,StefanoRaggi/Lean,JKarathiya/Lean,QuantConnect/Lean,jameschch/Lean,jameschch/Lean,StefanoRaggi/Lean,AlexCatarino/Lean,QuantConnect/Lean,StefanoRaggi/Lean,AlexCatarino/Lean,AlexCatarino/Lean,JKarathiya/Lean,jameschch/Lean,QuantConnect/Lean,AlexCatarino/Lean