tests/gdbus-method-invocation: Fix missing g_variant_new() argument

I noticed this when running the test on an Arm Morello system where varargs have bounds. g_variant_new() was trying to read an integer using va_arg(), but since there was no argument it resulted in a bounds errors there. On most other architectures this will just read whatever value is contained in the next argument register and is not something that ASan can detect, so it never resulted in test failures.

Split out from !3135 (closed)