Resolve session promise on test run cancellation (#1466)

When running a standard test session (not debugging) the xctest process
was terminated when the user hit the cancel test button, but the promise
to continue and mark the test run as ended was not resolved. This kept
the UI in a state that looked like the test was still running.

Author: plemarquand

src/TestExplorer/TestRunner.ts

@@ -776,6 +776,7 @@ export class TestRunner {
             // If the test run is iterrupted by a cancellation request from VS Code, ensure the task is terminated.
             const cancellationDisposable = this.testRun.token.onCancellationRequested(() => {
                 task.execution.terminate("SIGINT");
+                reject("Test run cancelled");
             });
 
             task.execution.onDidClose(code => {