refactor: account balance

[Daanvdplas]

• Main thing is that I combined the two tests relevant to the account balance (account balance update and deposit).
• The deposit tests were not checking the reserved balance so added that.
• Refactored the tests to common_functions as it much cleaner (separating unit tests to their own file).
• Changed the depositor for the atomic swap to the destination accounts.
• Left some comments to clarify how something works and why

[Daanvdplas]

Removed as it was used very inconsistently in the added code.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.98995% with 4 lines in your changes missing coverage. Please review.

Please upload report for BASE (frank/nfts-balance-deposit@e4020db). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pallets/nfts/src/impl_nonfungibles.rs	0.00%	4 Missing ⚠️

@@                      Coverage Diff                      @@
##             frank/nfts-balance-deposit     #421   +/-   ##
=============================================================
  Coverage                              ?   71.36%           
=============================================================
  Files                                 ?       72           
  Lines                                 ?    13555           
  Branches                              ?    13555           
=============================================================
  Hits                                  ?     9674           
  Misses                                ?     3608           
  Partials                              ?      273           

Files with missing lines	Coverage Δ
pallets/nfts/src/common_functions.rs	90.25% <100.00%> (ø)
pallets/nfts/src/features/atomic_swap.rs	90.78% <100.00%> (ø)
pallets/nfts/src/features/transfer.rs	83.68% <100.00%> (ø)
pallets/nfts/src/lib.rs	71.51% <ø> (ø)
pallets/nfts/src/tests.rs	99.91% <100.00%> (ø)
pallets/nfts/src/impl_nonfungibles.rs	22.12% <0.00%> (ø)

[chungquantin]

LGTM!