This has the SimObjects defined in "src/test_objects" only be compiled
into the gem5 binary if the Kconfig 'USE_TEST_OBJECTS" == 'y'. This
happens in two cases:
1. When 'ALL/gem5' is compiled via "build_opts".
2. When tests are run via "./tests/main.py".
Change-Id: I2330008fd7c7900de5f4de142b8ac89ef4e351ce