PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/NativeScript/blob/master/DevelopmentWorkflow.md#running-unit-tests.
• Tests for the changes are included - https://github.com/NativeScript/NativeScript/blob/master/WritingUnitTests.md.

What is the current behavior?

The image-cache tests are using the TKUnit.waitUntilReady method to wait for the expected result.

What is the new behavior?

The image-cache tests are using the async API.