[runtests] Accept a file path that specifies tests to run.

Also rename a few things for clarity.

Test: unit tests, manually tested with "runtests -f" on my linux host,
manually tested with no args on Fuchsia, which ran all tests in garnet,
and CQ will run runtests on all tests in Zircon.
Bug: IN-478 #comment
Bug: IN-527 #comment
Change-Id: I6f1485125130dadb70cefa921d80385aa0b1de33

[runtests] Create host version of runtests.

-Abstract the current runtests main() as a library method.
-Invoke with platform specific choices for
system/uapp/runtests/runtests.cpp
and
system/host/runtests/runtests.cpp

TESTED: runtests-utils-test passes as a host test; host runtests tested
by hand.

BUG: IN-358
Change-Id: Ic9bf53e4418e37d9856a5857b673f5cbd1fe2d4d