[libcxx] [test] Fix get/put long_double_ru_RU on Glibc, FreeBSD and Windows

Note, reducing ios.width() in put_long_double instead of using variable
padding, when using a variable width symbol. Some of those tests didn't
actually trigger any padding in the existing form, with a longer
currency symbol; reduce the width so there's no actual padding with the
slightly shorter currency symbol either.

The tests for the international currency symbol use the same amount of
padding on all platforms, so they still exercise the padding properly.

Differential Revision: https://reviews.llvm.org/D120317