gallium/auxiliary: Fix string matching

gallium/auxiliary: Fix string matching

Commit f69bc797e15fe6beb9e439009fab55f7fae0b7f9 did the following:

-        if format.layout in ('bptc', 'astc'):
+        if format.layout in ('astc'):

The intention was to go from matching either 'bptc' or 'astc' to
matching only 'astc'.

But the new code doesn't respect this intention any more, because in
Python `('astc')` is not a tuple containing a string, it is just the
string. (the parentheses are simply ignored)

That means we now match any substring of 'astc', for example 'a'.

This commit fixes the test to respect the original intention.

Fixes: f69bc797e15fe6beb9e4 "gallium/auxiliary: Add helper support for
                             bptc format compress/decompress"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>