RFR: 8324799: Use correct extension for C++ test headers [v2]

[Kim Barrett]

> Please review this change that renames some test .h files to .hpp.  These
> files contain C++ code and should be named accordingly.  Some of them contain
> uses of NULL, which we change to nullptr.
> 
> The renamed files are:
> 
> test/hotspot/jtreg/vmTestbase/nsk/share/aod/aod.h
> test/hotspot/jtreg/vmTestbase/nsk/share/jni/jni_tools.h  
> test/hotspot/jtreg/vmTestbase/nsk/share/jvmti/jvmti_tools.h
> test/hotspot/jtreg/vmTestbase/nsk/share/jvmti/JVMTITools.h
> test/hotspot/jtreg/vmTestbase/nsk/share/native/nsk_tools.h 
> 
> test/lib/jdk/test/lib/jvmti/jvmti_thread.h
> test/lib/jdk/test/lib/jvmti/jvmti_common.h
> test/lib/native/testlib_threads.h 
> 
> The #include updates were performed mostly mechanically, and builds would fail
> if there were mistakes.  The exception is test
> test/hotspot/jtreg/serviceability/jvmti/FollowReferences/FieldIndices/libFieldIndicesTest.cpp,
> which was added after I'd done the mechanical update, so was updated by hand.
> 
> The copyright updates were similarly performed mechanically. All but a handful
> of the including files have already had their copyrights updated recently,
> likely as part of JDK-8324681.
> 
> Thus, the only "interesting" changes are to the renamed files.
> 
> Testing: mach5 tier1

Kim Barrett has updated the pull request incrementally with two additional commits since the last revision:

 - add Oracle copyright
 - fix busted copyright text

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/18035/files
  - new: https://git.openjdk.org/jdk/pull/18035/files/4154005e..53f93950

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18035&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18035&range=00-01

  Stats: 3 lines in 2 files changed: 1 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/18035.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18035/head:pull/18035

PR: https://git.openjdk.org/jdk/pull/18035