RFR: 8343802: Prevent NULL usage backsliding

[Nizar Benalla]

Please review this patch to add a test that checks the hotspot sources and test files for usages of NULL.
It scans files in those directories, filtering out certain files as well as all `.c`, `.java` and `.jar` files in test sources. 

Before adding line 86 and excluding `os_windows.cpp`, the test failed with:


Error: 'NULL' found in /w/jdk/src/hotspot/os/windows/os_windows.cpp at line 4436:
    HMODULE hModule = NULL;
Error: 'NULL' found in /w/jdk/src/hotspot/os/windows/os_windows.cpp at line 4437:
    GetModuleHandleEx(GET_MODULE_HANDLE_EX_FLAG_UNCHANGED_REFCOUNT, NULL, &hModule);
java.lang.RuntimeException: Found usage of 'NULL' in source files. See errors above.
	at TestNoNULL.main(TestNoNULL.java:73)
	at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
	at java.base/java.lang.reflect.Method.invoke(Method.java:565)
	at com.sun.javatest.regtest.agent.MainActionHelper$AgentVMRunnable.run(MainActionHelper.java:333)
	at java.base/java.lang.Thread.run(Thread.java:1447)

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Commit messages:
 - Add a test to prevent NULL backsliding

Changes: https://git.openjdk.org/jdk/pull/23466/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23466&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343802
  Stats: 151 lines in 2 files changed: 150 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/23466.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23466/head:pull/23466

PR: https://git.openjdk.org/jdk/pull/23466