runtests.sh: build tests all together instead of one by one

Instead of building each test just before running it, build all
the tests together and then run them all together. This has two
advantages:

1. Building is faster because we don't have to start up soong and
   parse the ninja files for each test but only do it once at the
   beginning.
2. Test results appear all together and are easier to see in the
   scrollback buffer.

Test: ./system/netd/tests/runtests.sh
Test: ./system/netd/tests/runtests.sh unit
Test: ./system/netd/tests/runtests.sh utils
Change-Id: I3bb38aae5a1c05dca6a8bdbfbc98ed736854c96e