[LLDB] Fix sync issue in TestVSCode_launch.test_progress_events

This fixes flakiness in TestVSCode_launch.test_progress_events
vscode.progress_events some times failed to populate in time for
follow up iterations.

Adding a minor delay before the the for the loop fixes the issue.

Reviewed By: clayborg

Differential Revision: https://reviews.llvm.org/D99497