[libc++][tests] Fix a test testing wrong class

[pfusik]

This is just a small test cleanup: the directory and header comment said that it tests stringstream but it used istringstream.
The test was originally created this way: https://github.com/llvm/llvm-project/commit/1347d334518a3642243e97c7d173452443047e2f#diff-ea0557985f856925f1b019fcd64ee6d1ff0465ad8cd19a42b355e5b9c97589fe

[Mordante]

Nice catch! LGTM!

libcxx/test/std/input.output/string.streams/stringstream/stringstream.cons/move2.pass.cpp

 #include <string>
 #include <cassert>
 #include <cstddef>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
-    std::vector<std::istringstream> vecis;
-    vecis.push_back(std::istringstream());
-    vecis.back().str("hub started at [00 6b 8b 45 69]");
-    vecis.push_back(std::istringstream());
-    vecis.back().str("hub started at [00 6b 8b 45 69]");
-    for (std::size_t n = 0; n < vecis.size(); n++)
-    {
-        assert(vecis[n].str().size() == 31);
-        vecis[n].seekg(0, std::ios_base::beg);
-        assert(vecis[n].str().size() == 31);
-    }
+  std::vector<std::stringstream> vecss;
+  vecss.push_back(std::stringstream());
+  vecss.back().str("hub started at [00 6b 8b 45 69]");
+  vecss.push_back(std::stringstream());
+  vecss.back().str("hub started at [00 6b 8b 45 69]");
+  for (std::size_t n = 0; n < vecss.size(); n++) {
+    assert(vecss[n].str().size() == 31);
+    vecss[n].seekg(0, std::ios_base::beg);
+    assert(vecss[n].str().size() == 31);
 
   return 0;
 }