tests: set output_file to a valid value

If fdopen(3) fails then ctx->output_file is NULL and should not be used
in cleanup_test_output() called next.

Problem detected by the Coverity tool
CID 1042543 (#1 of 1): Dereference after null check (FORWARD_NULL)9.
var_deref_model: Passing "ctx" to function
"cleanup_test_output(libusbx_testlib_ctx *)", which dereferences null
"ctx->output_file".