shlwapi/tests: Fix test_StrFromTimeIntervalA() to print the badly formatted string in case of a failure.

This is similar to 5d1d07ab. We need to flush
to ensure resource updates are visible in other contexts, since otherwise GL
doesn't make any ordering guarantees between contexts.

rpcrt4: The named pipe can be used for multiple operations at the same time, so use overlapped operations for ReadFile and WriteFile.