Refactor unit tests without depending on Guava

[kongleong86]

Refactor unit tests without depending on Guava external library

• Using less libraries to do unit tests will help on code maintainability and future libraries upgrade.

For detailed contributing instructions see https://github.com/iluwatar/java-design-patterns/wiki/01.-How-to-contribute

[robertvolkmann]

@kongleong86 Your branch has conflicts. Could you resolve them?

[sonarqubecloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.0% Duplication

[kongleong86]

Please resolve the conflicts and we can proceed with the merge

Conflicts resolved.

[robertvolkmann]

@iluwatar could you please merge? It would simplify #2357.

[iluwatar]

Looks good! Thank you for the contribution 🎉

@all-contributors please add @kongleong86 for code

[allcontributors[bot]]

@iluwatar

I couldn't determine any contributions to add, did you specify any contributions? Please make sure to use valid contribution names.

@kongleong86 already contributed before to code

[iluwatar]

@all-contributors please add @robertvolkmann for review

[allcontributors[bot]]

@iluwatar

@robertvolkmann already contributed before to review