HandleGetExtensionSuccessfully creates an HTTP handler at `/extensions/agent` that responds with a JSON payload corresponding to SingleExtension.

HandleListExtensionsSuccessfully creates an HTTP handler at `/extensions` on the test handler mux that response with a list containing a single tenant.

GetOutput provides a single Extension result.

ListOutput provides a single page of Extension results.

ExpectedExtensions is a slice containing the Extension that should be parsed from ListOutput.

ListedExtension is the Extension that should be parsed from ListOutput.

SingleExtension is the Extension that should be parsed from GetOutput.