RFR: 8324799: Use correct extension for C++ test headers

[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

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

Commit messages:
 - update new test
 - update copyrights
 - fix jvmti README
 - rename NULLs in jvmti_common.hpp
 - rename jvmti_common.h
 - rename NULLs in jvmti_thread.hpp
 - rename jvmti_thread.h
 - rename NULLs in testlib_threads.hpp
 - rename testlib_threads.h -- no copyright
 - rename nsk_tools.h -- no copyright
 - ... and 5 more: https://git.openjdk.org/jdk/compare/16d917a8...4154005e

Changes: https://git.openjdk.org/jdk/pull/18035/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18035&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324799
  Stats: 1535 lines in 731 files changed: 133 ins; 133 del; 1269 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