[lit] Fix bug when using Python3 where a failing test would not show

the script when running a ShTest with an external or internal shell.

This bug is caused by use of the ``map`` function in Python 3 which
returns an iterable (rather than a list in Python 2). After the iterable
is exhausted it won't return any more output and consequently when
``_runShTest()`` tries to access the ``script`` which has already been
iterated over it is empty. Converting to a list immediatley after
calling ``map()`` fixes this.

This fixes the ``tests/shtest-format.py`` test when running under
Python3 which was previously failing.

llvm-svn: 253556