test(bmc): Handle Relay Message when neither source nor destination is ICON chain

[izyak]

Closes #776

Checklist:

• [x] I have performed a self-review of my own code
• [x] I have documented my code in accordance with the documentation guidelines
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have run the unit tests
• [x] I only have one commit (if not, squash them into one commit).
• [x] I have a descriptive commit message that adheres to the commit message guidelines

Please review the CONTRIBUTING.md file for detailed contributing guidelines.

[codecov[bot]]

Codecov Report

Merging #777 (f77fc7c) into main (9af33da) will increase coverage by 0.07%. The diff coverage is n/a.

@@             Coverage Diff              @@
##               main     #777      +/-   ##
============================================
+ Coverage     38.20%   38.27%   +0.07%     
- Complexity      801      804       +3     
============================================
  Files           115      115              
  Lines         11161    11161              
  Branches        941      941              
============================================
+ Hits           4264     4272       +8     
+ Misses         6579     6574       -5     
+ Partials        318      315       -3     

Impacted Files	Coverage Δ
...java/foundation/icon/btp/bmc/BTPMessageCenter.java	81.38% <0.00%> (+0.84%)	:arrow_up:
...main/java/foundation/icon/btp/bmc/InitMessage.java	92.30% <0.00%> (+5.12%)	:arrow_up: