Extend validate_failures.py to run outside the build directory.

This patch adds three new arguments to validate_failures.py so
it can be used outside the build directory:

--ignore_missing_failures
    When a failure is expected in the manifest but it is not found in
    the actual results, the script produces a note alerting to this
    fact. This means that the expected failure has been fixed, or it
    did not run, or it may simply be flaky.

    With this option, one can ask the script not to show the missing
    failures.

--manifest
    Name of the manifest file to use.  By default, the script will
    look for the manifest file in the source directory associated with
    this build.  With this option, one can point to any arbitrary
    manifest file.  I renamed the old --manifest flag to
    --produce_manifest.

--results
    Space-separated list of .sum files with the testing results to
    check. The only content needed from these files are the lines
    starting with FAIL, XPASS or UNRESOLVED.

From-SVN: r188217