restrict isRequestComplete to only the related oracle response

[trmid]

Changed isRequestComplete to be consistent with isRequestFailed such that it only considers the oracle response that resulted from the request. This provides consistency across the implementation while sacrificing the Witnet order backfill behaviour for requests; meaning that a request will only be considered complete if it's oracle response returned successfully.

The original issue points out a problem that arises from the inconsistent interface where an RNG request can be considered both failed and completed at the same time. This change enforces a consistent interface between the two so bots can make decisions in a more predictable environment.

[linear]

GEN-1767 [27] Draws can be retried even if a random number is available or the current draw has finished

[github-actions]

LCOV of commit 2ddd97f during Tests with 100% Coverage #56

Summary coverage rate:
  lines......: 100.0% (34 of 34 lines)
  functions..: 100.0% (16 of 16 functions)
  branches...: no data found

Files changed coverage rate:
                   |Lines       |Functions  |Branches    
  Filename         |Rate     Num|Rate    Num|Rate     Num
  =======================================================
  src/RngWitnet.sol| 100%     26| 100%    12|    -      0