Add getAttestationDeltas performance test

[twoeths]

part of #2046

• Fix and restructure the current performance tests
• Add getAttestationDeltas.test.ts showing that it may finish in more than 1s as in the profiler:

#	minTime (ms)	maxTime (ms)	average (ms)
1st	98	1375	130
2nd	97	699	131

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[twoeths]

I'm not familiar with assembly-script. Since getAttestationDeltas mainly doBigIntUintType({byteLength: 8}) operations, can we use u64 type of assembly-script and implement the function in wasm @ChainSafe/eth2-0 ? at least we want it to be predictable.

[mpetrunic]

I'm not familiar with assembly-script. Since getAttestationDeltas mainly doBigIntUintType({byteLength: 8}) operations, can we use u64 type of assembly-script and implement the function in wasm @ChainSafe/eth2-0 ? at least we want it to be predictable.

We could but not sure if worth it since I think that method is removed in HF1 and replaced with https://github.com/ethereum/eth2.0-specs/blob/dev/specs/lightclient/beacon-chain.md#get_flag_deltas

It might be better to write new method in assemblyscript later on