test(xyk): add missing tests

[Roznovjak]

Description

Add missing tests found by code coverage tools.

[codecov]

Codecov Report

Merging #294 (f652077) into master (00c53a4) will increase coverage by 0.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
+ Coverage   14.82%   15.27%   +0.45%     
==========================================
  Files          24       20       -4     
  Lines        6970     6776     -194     
==========================================
+ Hits         1033     1035       +2     
+ Misses       5937     5741     -196     

Impacted Files	Coverage Δ
pallets/xyk/src/lib.rs	84.01% <ø> (+2.80%)	⬆️
pallets/exchange/src/weights.rs
pallets/xyk/src/weights.rs
pallets/claims/src/weights.rs
pallets/transaction-multi-payment/src/weights.rs

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