init: clean up subcontext_test

subcontext_test had been failing due to setexeccon() failing to
transition to vendor_init context.  This is a good thing as nothing
other than init should be able to transition into this context.

I don't want to add code to skip the setexeccon() call only for the
tests, so I instead call setexeccon() with the return value of
getcon().  This works however only for root, so these tests are
skipped for non-root.

Test: init unit tests
Change-Id: I8a415599e0ec5506511202f7f5018c0e5265837d