[libcxx] [test] Fix the get_weekday test on glibc

This test feeds in the expected utf8 form of weekdays in various
languages, trying to match what libc++ has gathered internally
from `strftime()`. On glibc, the ru_RU.UTF-8 representation of the
tested weekday is spelled with upper case, while the existing
test reference is lower case.

Differential Revision: https://reviews.llvm.org/D118237