Asset handler tests improvement

[serhii-temchenko]

In this PR:

• Common implementation in thea related tests moved to separate private functions to be reused.
• Extended amount of tests to cover more cases
• Added functions with todo's inside for missing cases

[codecov]

Codecov Report

Base: 53.79% // Head: 54.24% // Increases project coverage by +0.44% 🎉

Coverage data is based on head (d851dab) compared to base (dc169e4).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@                 Coverage Diff                  @@
##           fx-asset-handler     #602      +/-   ##
====================================================
+ Coverage             53.79%   54.24%   +0.44%     
====================================================
  Files                    20       20              
  Lines                  1461     1462       +1     
====================================================
+ Hits                    786      793       +7     
+ Misses                  675      669       -6     

Impacted Files	Coverage Δ
pallets/asset-handler/src/lib.rs	76.09% <100.00%> (+3.05%)	⬆️

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