fix(`aws-kms-adapter`): flaky test fixed

[freemanzMrojo]

Description

When running tests over testnet concurrently, sometimes we try to add a recipient when it has been added already by one of the tests. This PR deals with that.

Closes #1491

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

SDK test suite.

Test Configuration:

• Node.js Version: 20.18.0
• Yarn Version: 1.22.19

Checklist:

• [x] My code follows the coding standards of this project
• [x] I have performed a self-review of my code
• [x] I have commented on my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] New and existing integration tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have not added any vulnerable dependencies to my code

[github-actions[bot]]

Test Coverage

Summary

Lines	Statements	Branches	Functions
	98.99% (4336/4380)	97.61% (1390/1424)	98.99% (890/899)

Title	Tests	Skipped	Failures	Errors	Time
core	809	0 :zzz:	0 :x:	0 :fire:	2m 23s :stopwatch:
network	726	0 :zzz:	0 :x:	0 :fire:	5m 0s :stopwatch:
errors	42	0 :zzz:	0 :x:	0 :fire:	17.918s :stopwatch:
logging	3	0 :zzz:	0 :x:	0 :fire:	18.822s :stopwatch:
hardhat-plugin	19	0 :zzz:	0 :x:	0 :fire:	1m 2s :stopwatch:
aws-kms-adapter	23	0 :zzz:	0 :x:	0 :fire:	1m 24s :stopwatch:
ethers-adapter	5	0 :zzz:	0 :x:	0 :fire:	1m 14s :stopwatch:
rpc-proxy	37	0 :zzz:	0 :x:	0 :fire:	1m 5s :stopwatch: