Fix init test cases from DexClassLoaderTest.

Because the tests cases are executed in the same runtime and the class
linker may cache generated optimized files, init tests need to use their
own optimized directories (so that they can reliable test the number of
generated files).

Bug: 16806146

(cherry picked from commit 58c130c1)

Change-Id: I8111cc7ea640f3bc94821fcd4380acfc078c746f