llvmpipe: don't test for unsupported formats in lp_test_format

llvmpipe: don't test for unsupported formats in lp_test_format

Removing the fake format helpers (1c7d0a6aa4f5cb38af7e281e1e5437cd1a20f781)
caused this to fail. These formats were never supported, but previously
they would have asserted in the generated jit functions (which, due to lack
of test cases for these formats, were never called) whereas we now assert when
trying to build the jit function. So, skip them completely.

This fixes https://bugs.freedesktop.org/show_bug.cgi?id=93092

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>