Fallback to working dir for native lib dir in tests (elastic#133422)

When finding the platform dir to pass to jna we check for
system properties which are either set by internal test gradle code or
or the cli shell script. If neither of thse are set, we are in test code
for an external plugin. This commit fixes the code to use the working
dir in this fallback case. Previously we would have created a bogus
path. Both are the same: external plugin tests can't load ES native libs
since they don't have them available.

Author: rjernst

libs/native/src/main/java/org/elasticsearch/nativeaccess/lib/LoaderHelper.java

@@ -26,7 +26,16 @@ private static Path findPlatformLibDir() {
             return Paths.get(path);
         }
 
-        Path homeDir = Paths.get(System.getProperty("es.path.home"));
+        String homeDirPath = System.getProperty("es.path.home");
+        if (homeDirPath == null) {
+            // if we don't have the native libs passed in for tests, and we don't have the home dir set
+            // then we must be in a test for an external plugin. there's nothing we can do to get the
+            // native libs, they don't exist, so just return the working dir, it will fail if the test
+            // actually tries to load a library
+            return Paths.get("");
+        }
+
+        Path homeDir = Paths.get(homeDirPath);
         Path platformDir = homeDir.resolve("lib/platform");
 
         String osname = System.getProperty("os.name");