[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

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.virtuals/overflow.pass.cpp

@@ -38,7 +38,7 @@ struct testbuf
 
 int main(int, char**)
 {
-    // sanity check
+    // check overflow
     {
         testbuf<char> tb("");
         tb.overflow();

libcxx/test/std/input.output/string.streams/stringbuf/stringbuf.virtuals/pbackfail.pass.cpp

@@ -36,7 +36,7 @@ struct testbuf
 
 int main(int, char**)
 {
-    // sanity check
+    // check pbackfail
     {
         testbuf<char> tb("");
         tb.pbackfail();