Add --check-shebang to bisect-runner.pl, to avoid mistakes with #! lines

Add --check-shebang to bisect-runner.pl, to avoid mistakes with #! lines

This allows bisect.pl to instruct bisect-runner.pl to check that the user's
testcase is not a script with a #! line which will run an installed perl.
This rapidly catches the potential error of specifying a test case that will
not actually be tested with the bisect-build perl. Without this, the user
may end up with an unexpected failure to bisect, and little clue as to what
the actual problem is.