[gdb/testsuite] Use unique files in gdb.dwarf2/dw2-lines.exp

While debugging a problem in gdb.dwarf2/dw2-lines.exp, I realized that the
test-case generates all executables and associated temporary files using the
same filenames.

Fix this by adding a new proc prefix_id in lib/gdb.exp, and using it in the
test-case.

Tested on x86_64-linux.