There are a number of header files in test/hotspot/jtreg that have ".h" extensions but are actually C++ code, or are at least only included by C++ files. Rename them, to follow normal HotSpot style.

Also rename uses of NULL to nullptr in the renamed headers, as that change is not included in the in-progress JDK-8324681. The PR for that RFE only changed properly named C++ files.