[compiler-rt][MSAN][test] Update buffersize for UTF-8 to C convert

This is exposed by https://reviews.llvm.org/D83486.
When the host is UTF8, we may get n >10, causing assert failure.
Increase the buffersize to support UTF-8 to C conversion.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D83719