[approvaltest] Ignore dynamic fields from sort fields

[lahsivjar]

The approval tests doesn't handle dynamic fields correctly, the fields are sorted before dynamic is handled. Due to this, 2 issues could happen:

1. If a field is passed as dynamic but it is also in the sort fields then the resulting sort would be indeterministic.
2. If all the sort fields are identical then the fields are compared using bytes.Compare this will again result in indeterministic sorts if the dynamic fields are randomly generated.

Both these issues are fixed in this PR by rewriting the dynamic fields before sorting and comparing for approvals.

Required to fix the failures in elastic/apm-server#13196

[axw]

Could this PR be simplified by pulling the rewrite loops out of approveEventDocs and approveFields? i.e. change ApproveEvents and ApproveFields to first rewrite, then sort, and finally approve.

[lahsivjar]

Could this PR be simplified by pulling the rewrite loops out of approveEventDocs and approveFields?

@axw This was my first thought and I spent some time into trying to refactor code as per this but the code I came up with was not optimum and required copying the raw fields and the raw source both in-memory. My previous effort was based on not decoding the data first but I think we might be able to do this if we decode to JSON first and then parse dynamic fields. I will give this a shot.

[lahsivjar]

@axw Fixed the logic to rewrite before sort. PTAL.

[axw]

Looks good, just one query

[axw]

Is this going to work for _source as well as fields? We might need to switch based on whether it's ApproveEvents vs. ApproveFields

[lahsivjar]

@axw You are right, it wouldn't have worked with ApproveEvents, I have fixed it now. Thanks for catching this.