TestFwdBackUnsafe hangs on Windows with MSVC VS2017
Description
Steps:
Checkout trunk [r41094].
Build with MSVC (VS2017) for 'x64' and 'Release'.
Run tests with `icucheck.bat x64 Release`.
`cintltest.exe` hangs running the utf8tst function "`TestFwdBackUnsafe`".
