gas/testsuite: adjust another test for case insensitive file systems

As 1fafeaac8503eea2f61c3a35f0eef183b7e7cc65, "line.s" and "Line.s" are
identical in case insensitive file systems. Thus, gas doesn't trigger
an input file switch.

gas/ChangeLog:

        * testsuite/gas/elf/dwarf-5-macro.s: Change Line.s to Line2.s.