[libcxx][NFC] Inclusive language: remove use of sanity check

from libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.virtuals/

[NFC] As part of using inclusive language within the llvm project, this
patch rewords comments to remove sanity check.

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D124391