Document the testing approach once a runner lands

[sshdopey]

Context

When the test runner is added, CONTRIBUTING should explain how to write and run tests so coverage grows consistently.

Scope

• Add a testing section: structure, helpers, and how to run unit and end-to-end tests.

Acceptance criteria

• CONTRIBUTING explains how to write and run tests.

Pointers

CONTRIBUTING.md.

[Adeyemi-cmd]

@Adeyemi-cmd has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

can i please work on this

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Adeyemi-cmd to this issue.

[Bilalishaq7]

@Bilalishaq7 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Gooday maintainer, I'll like to take on this issue and i'll submit a PR shortly

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Bilalishaq7 to this issue.