RFR (trivial) 8249940: Remove unnecessary includes of jni_util.h in native tests

[David Holmes]

Bug: https://bugs.openjdk.java.net/browse/JDK-8249940
webrev: http://cr.openjdk.java.net/~dholmes/8249940/webrev/

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.

Thanks,
David
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