[fx] Have run-host-tests consult tests.json

`fx run-host-tests` relied on all host tests being available in a
single directory. With the consultation of tests.json for host
test locations, this is no longer necessary.

Though it might not be ideal, this preserves CLI of the existing
run-host-tests, with the exception of removing -v for verbosity,
as that may always be passed after `--`.

Test:
fx run-host-tests containers_cpp_unittests cpuperf_raw_printer_test
fx run-host-tests containers_cpp_unittests -- --gtest_filter=VectorBuffer*
fx run-host-tests nonexistent
fx run-host-tests containers_cpp_unittests nonexistent

Bug: IN-849 #done
Bug: DNO-456 #done

Change-Id: Ib3872c86e089f70ae3d11d91cc3d983d17eaac23