meson.build: add a target for cargo test --no-run
Because cargo is cargo, running cargo test will pull everything down again and build it with whatever slightly different options are set. This will cause timeouts on the meston test run and/or other issues if a network connection is no longer available during the build/test stage.
Work around this by adding another target that runs "cargo test --no-run". This way everything is built and ready to go by the time we get to meson test, though at the cost of longer build times (and possibly duplicate error message?).
I'm not sure this is what we want or whether it should be hidden behind a config option. Alternatively we can just increase the timeout to something ridiculous (see first patch in this MR). If this is merged, the first patch of this MR isn't required.
Either way: motivation was the build failing in koji with a timeout.