[runtests] Remove memory leak and redundant print-out.

Tested: ran binary and extraneous print-out was indeed gone.
Change-Id: I05421f5b9928270cc053b97207d1e74042daa8ac

[runtests] Fix macOS breakage.

stdlib.h and unistd.h were deleted in a recent change when they
shouldn't have been. mkdtemp is included from stdlib.h and on macOS
requires unistd.h

Change-Id: I696f5a1a13758ed993217e07163b7aa3d919a7ae

[runtests] Introduce scoped test directories to runtests-utils-test

Currently, the same directory is being used for file I/O across all test
cases, which could introduce unwanted dependencies. Here we introduce
ScopedTestDir to provide a directory for test scoped to each test case.

BUG: IN-478
Change-Id: If9591f6b8334b323783d5fafa84709a20be70c2b

[runtests] Define PosixRunTest() for running host tests.

Note: runtests-utils-test passes as a host test.

BUG: IN-358
Change-Id: I9f0380b4d3c38c6fea5950638e5eb0cfaa1ab3d3