Don't re-use arttest when calling loadLibrary.

When Android's build environment variables are set with envsetup.sh,
the test "works" ok, by getting a LinkageError because two class loaders
try to load the same library. I guess that is the reason for the
if (ExceptionCheck()) after the loading.

However, if the environment variables are set manually, there are
cases where the paths provided between a Java loadLibrary, and a
native loadLibrary are different, so we end up loading the library twice.
This makes the assertion line 32 fail on the second JNI_OnLoad call.
In my particular environment, ANDROID_BUILD_TOP was something lie
/foo/bar/..//.

This change stops re-using the same library, and makes the expected
outcome constant: the native call of loadLibrary with a non exist
library must throw a LinkageError.

Change-Id: I8721a03715e099c55fb8b2b87813f1e772c8e83d