test.sh: add support for running on Apple Silicon

[darccio]

What does this PR do?

Adds support for Apple Silicon by setting the appropriate environment variable to force Docker images' platform to be x86 64 (amd64).

Motivation

Easily run integration tests locally.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[darccio]

Thanks @ahmed-mez! 🥷