[zircon][build] Add tests_in_image GN build argument

Setting tests_in_image=false is the rough equivalent of passing
DISABLE_UTEST=true in the old makefile system.

Bug: ZX-3525 #done
Test: manual build with --args=tests_in_image=false and tools/zbi -tv inpsection
Change-Id: I1e562cfed7a264dfd6f15cf6c5815ef37feff704