Revert "[utest] Don't hard-code /boot location for zircon test files"

This reverts commit 1651ebe6.

Reason for revert: The change rolled too much into one commit and did not get anything green.  It needs to be split up and reviewed properly.

Original change's description:
> [utest] Don't hard-code /boot location for zircon test files
> 
> This change fixes the zircon tests that were failing in the `bringup`
> build.
> 
> The tests were failing because they had hardcoded paths at "/boot/..."
> which failed when the tests were run from "/system". The fix is for
> runtests to pass a TEST_ROOT_DIR environment variable, which is
> "/system" or "/boot" depending on the location of the test. The tests
> use this variable instead of hardcoding the path.
> 
> Bug: BLD-463 #comment fix broken tests
> Change-Id: Id6478f68600a55f13dc8c0bde19253c909a03694

TBR=mcgrathr@google.com,geb@google.com,shayba@google.com
Bug: BLD-463

Change-Id: I9d5a296eee311d48ac45a7dd8339d889aca18d4d