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

[quinnp]

Summary

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.

[philnik]

LGTM

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

typename base::int_type
         overflow(typename base::int_type c = base::traits_type::eof())
         {++overflow_called; return base::overflow(c);}
 
     void pbump(int n) {base::pbump(n);}
 };
 
 int main(int, char**)
 {
-    // sanity check
+    // check overflow
     {
         testbuf<char> tb("");
         tb.overflow();
     }
     {
         testbuf<char> sb("abc");
         assert(sb.sputc('1') == '1');
         assert(sb.str() == "1bc");

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

typename base::int_type
         pbackfail(typename base::int_type c = base::traits_type::eof())
         {return base::pbackfail(c);}
 
     void pbump(int n) {base::pbump(n);}
 };
 
 int main(int, char**)
 {
-    // sanity check
+    // check pbackfail
     {
         testbuf<char> tb("");
         tb.pbackfail();
     }
     {
         testbuf<char> sb("123", std::ios_base::in);
         assert(sb.sgetc() == '1');
         assert(sb.snextc() == '2');