[lit] Fix the return code for "not not" after evaluating "not" internally

This fixes cases where "not not <command>" is supposed to return
only the error codes 0 or 1, but after efee5792,
it passed the original error code through.

This was visible on AIX in the shtest-output-printing.py testcase,
where 'wc' returns 2, while it returns 1 on other platforms, and the
test required "not not" to normalize it to 1.