feat(multipayment)!: use fallback in set_currency

[Roznovjak]

Description

Use fallback account and price in set_currency() if the corresponding pool does not exist.
Add missing tests found by code coverage tools.

[codecov]

Codecov Report

Merging #93 (f37a3de) into master (8e88a07) will increase coverage by 0.66%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
+ Coverage   86.72%   87.39%   +0.66%     
==========================================
  Files          17       17              
  Lines        1793     1793              
==========================================
+ Hits         1555     1567      +12     
+ Misses        238      226      -12     

Impacted Files	Coverage Δ
pallets/transaction-multi-payment/src/lib.rs	80.64% <100.00%> (+9.67%)	⬆️

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[enthusiastmartin]

looks ok. However there is a change in xyk pallet which has introduced trading limits. So the test would need some adjustments when merged.

[enthusiastmartin]

Looks ok.

However the withdrawal is not yet enabled in the runtime. It needs to be set to Yes.