[libc++] Fix forgotten fclose() in unit test

Thanks to Andrey Maksimov for the patch.
Differential Revision: https://reviews.llvm.org/D58732

llvm-svn: 355162

libcxx/test/std/input.output/stream.buffers/streambuf/streambuf.virtuals/streambuf.virt.put/xsputn.PR14074.pass.cpp

@@ -34,6 +34,7 @@ static std::size_t count_bytes(char const* filename) {
     std::size_t count = 0;
     while (std::fgetc(f) != EOF)
         ++count;
+    std::fclose(f);
     return count;
 }