Move the implementation of --validate from bisect.pl to bisect-runner.pl

Move the implementation of --validate from bisect.pl to bisect-runner.pl

--validate sets a default testcase by assigning to @ARGV if its empty. It
makes more sense to do this in bisect-runner.pl, as that processes options
fully, unlike bisect.pl, which passes most through. Hence bisect.pl doesn't
know if some elements of @ARGV are actually options, and hence no testcase
has been supplied, and hence the default is needed.

This change permits the use of --validate with build options such as -D to
work as expected.