A number of native tests in hotspot and jdk include the jni_util.h header file which is part of the sources for libjava and not part of the testing framework, nor an exported interface for the JDK. This seems to have occurred through copy-and-paste when creating the tests as the include is not needed.

test/hotspot/jtreg/runtime/jni/FindClass/libbootLoaderTest.c
test/hotspot/jtreg/runtime/jni/registerNativesWarning/libregisterNativesWarning.c
test/hotspot/jtreg/runtime/jni/terminatedThread/libterminatedThread.c
test/jdk/java/lang/ClassLoader/nativeLibrary/libnativeLibraryTest.c
test/jdk/java/lang/ProcessBuilder/checkHandles/libCheckHandles.c
test/jdk/jdk/internal/loader/NativeLibraries/libnativeLibrariesTest.c

There is one test that includes jni_util.h and uses the utility function declared there:
./jdk/java/lang/String/nativeEncoding/libstringPlatformChars.c
so that is left as-is.