[llvm-mc] Fix line_end_with_space.test

Currently, this test doesn't test what it was originally intended to
(that lvm-mc doesn't crash when disassembling a line that ends with a
space). This patch readds the space to the input so that this
functionality is actually tested and adds a comment describing what the
test does. The space seems to have been inadvertently removed in
c8d57831 and then never added back
in.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D147207