tests/xdg_shell: Expect code unconfigured_buffer
This makes the xdg_surface_disallows_buffer_before_first_configure
test require a unconfigured_buffer
code to pass, both miral and anvil fail with "Failed to receive protocol error UnconfiguredBuffer on xdg_surface".
Should expect_failure_code
take a string argument to add context to the failures? It could give the error "Buffer commit should fail (Failed to receive protocol error UnconfiguredBuffer on xdg_surface)"
Is there any reason to keep expect_failure
around, will there be tests where we're unsure of what the error code will be?