[Identity] Add testing to VSCodeCredential

[sophiajt]

Is your feature request related to a problem? Please describe.
As a follow-on from #7994, we need to add some way to test VSCodeCredential to ensure it continues to work. We could take a page from the work on AzureCLICredential and mock the connection to keytar.

Describe the solution you'd like
Mock a keytar connection that successfully returns a known token, and ensure that token is valid on the credential side.

[witemple-msft]

Implemented in identity-vscode: https://github.com/Azure/azure-sdk-for-js/blob/main/sdk/identity/identity-vscode/test/public/node/visualStudioCodeCredential.spec.ts#L35