Unfortunately just adding suppressions isn't enough; the build of Expat
we have in our CI system does not have frame pointers, so ASan's fast
unwinder can't see through it. This means that the suppressions we've
added won't be taken into account.

For now, disable the fast unwinder for the Xwayland test only. Disabling
it globally is not practical as it massively increases the per-test
runtime, so here (to avoid polluting the build system), we use a
per-test wrapper to selectively choose the unwinder.

Signed-off-by: Daniel Stone <daniels@collabora.com>