RFR: 8346139: test_memset_with_concurrent_readers.cpp should not use <sstream>

[Kim Barrett]

Please review this change to test_memset_with_concurrent_readers.cpp to use
HotSpot's stringStream instead of std::string_stream to build the error
message when a failure is detected.

Also removed the include of <string.h>, which is one of the standard headers
we expect to be included by globalDefinitions.

Testing: mach5 tier1
Locally patched the test to fail and ran it, checking the output.

-------------

Commit messages:
 - no iostream in test

Changes: https://git.openjdk.org/jdk/pull/22725/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22725&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346139
  Stats: 23 lines in 1 file changed: 2 ins; 6 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/22725.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22725/head:pull/22725

PR: https://git.openjdk.org/jdk/pull/22725