[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible...

[libcxx] [test] Fix MSVC warning C4244 "conversion from 'X' to 'Y', possible loss of data", part 4/7.

Change char to long and remove some char casts. This preserves test coverage for tuple's
heterogeneous comparisons, while avoiding int-to-char truncation warnings.

Fixes D27541.

llvm-svn: 289108