Add the missing synchronization in old_tests/async.cpp - #1548

Add the missing synchronization in old_tests/async.cpp#1548

Yexuan Xiao (YexuanXiao) wants to merge 3 commits into

microsoft:mastermicrosoft/cppwinrt:masterfrom

YexuanXiao:fix-old-async-test-upstreamYexuanXiao/cppwinrtplus:fix-old-async-test-upstreamCopy head branch name to clipboard