tests: Verify that wayland_scanner can catch bad identifiers

The test runs wayland_scanner on a set of XML protocol files which
have malformed element names, and confirms that an error is produced
and indicates the correct line.

Copyright notifications are not included in the test files, as
they are not code; of course, the project license still applies.

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>