[jdk21u-dev] RFR: 8324799: Use correct extension for C++ test headers

[Goetz Lindenmaier]

I would like to backport this to keep the tests up to date and simplify later backports as 8202100: Merge vm/share/InMemoryJavaCompiler w/ jdk/test/lib/compiler/InMemoryJavaCompiler

All but three files patched clean, edited two files additionally, see extra commit.

test/hotspot/jtreg/serviceability/jvmti/GetClassFields/FilteredFields/libFilteredFieldsTest.cpp
Added by "8318626: GetClassFields does not filter out ConstantPool.constantPoolOop field", already labeled fix-no
Omitted.

test/jdk/java/foreign/enablenativeaccess/org/openjdk/foreigntest/unnamed/libLinkerInvokerUnnamed.cpp
Added by "8312522: Implementation of Foreign Function & Memory API" in 22.
Omitted.

test/jdk/java/foreign/stackwalk/libAsyncStackWalk.cpp
Later change "8327460: Compile tests with the same visibility rules as product code" already backported. Resolved.

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Commit messages:
 - Additional edits
 - Backport 998d0baab0fd051c38d9fd6021628eb863b80554

Changes: https://git.openjdk.org/jdk21u-dev/pull/1792/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1792&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324799
  Stats: 1538 lines in 731 files changed: 134 ins; 134 del; 1270 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1792.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1792/head:pull/1792

PR: https://git.openjdk.org/jdk21u-dev/pull/1792