Reorganize function tests

[GiedriusS]

#142 (review)

With lots of functions, it's hard to understand what is being tested and what is not. Reorganize the tests to make them clearer.

[fatsheep9146]

I'd love to help with this, what way of reorganizing do you suggest? @GiedriusS @fpetkovski

[fpetkovski]

First thing I would suggest is merging instant and range query tests, so that we add test cases in only one place. I would look into how we can define multiple expressions in a test case for a single load. This way we don't need to repeat the loaded series over and over when we add a new function.

[GiedriusS]

I was thinking maybe we could cover more cases in the fuzzer https://github.com/thanos-community/promql-engine/blob/main/engine/enginefuzz_test.go#L21 and we could remove simple cases from that testing function #142 (review).

[rakeshkumarnahak]

/assign

[MichaHoffmann]

I wonder, why does the enginefuzz_test.go not generate NaN or Inf values that seems like it could produce interesting edge cases!

[fpetkovski]

Up until recently we had no support for NaN comparisons since there are many different ways to represent a NaN in go. We could it it now though, and use cmpopts.EquateNaNs() as in https://github.com/thanos-community/promql-engine/blob/main/engine/engine_test.go#L1565.