builder: better handle some index parsing errors

Add a new lexer token, unused in the grammar, for the unknown lines in
index files; this should allow to better handle such kind of parsing
errors, removing the need to exit() directly (and leave things in an
unclean state).
builder: Don't use configure to generate test-index file.

The file contained no replacement patterns so generating it
with configure did nothing.
builder: Rearrange EXTRA_DIST in alphabetical order.

No functional change.
builder: Add test of virt-index-validate.