Split image_test and oatdump_test.

With sufficient -j, splitting the oatdump_test makes gtests
finish faster, splitting image_test makes valgring tests
finish faster.

make -j 48 valgrind-test-art-host-gtest
  - before: #### make completed successfully (05:20:16 (hh:mm:ss)) ####
  - after: #### make completed successfully (03:05:43 (hh:mm:ss)) ####
Note that the new image_*test variants are still running
long after the rest of the tests is finished. Further
splitting may still be beneficial for host tests. Target
tests with lower -j parameter do not benefit that much.

Test: make -j 48 valgrind-test-art-host-gtest
Bug: 36849303
Change-Id: I2b887e6b3266f9d3efd7b4d71dde0fcf012829d2