Fix up contracts pallet tests

[ascjones]

• Revert contracts put_code test to pure code (not using the subxt_test macro), for better readability and debuggability (is that a word?)
• Add runtime for the node template with the contracts module added.
• Add integration-tests feature for tests which require a running substrate node, instead of just #[ignore]ing them

[dvdplm]

LGTM.

I'd like some docs on how to run the integration-tests tests, but it's ok to defer to a separate PR if you prefer.

EDIT:
I had this in the README for a draft PR, maybe you can adapt it:

+## Running tests
+The normal `cargo test` command works for most tests. There are a few client
+related tests that are `#[ignore]`'d by default. To run them manually, follow
+these steps:
+
+1. Build the test node: `cargo build --release -p test-node`
+1. In the `client` directory, run `./gen-chain-spec.sh` to generate a chain spec file called `dev-chain.json`
+1. Run `run.sh` to start the test node
+1. Uncomment the `#[ignore]`'d test(s) you wish to run
+1. Run the test with e.g. `cargo test -p client test_light_client`

[ascjones]

Cool I think we can do that as a separate PR to unify the testing features and #[ignore]. And currently the tests under integration-tests require a customised template node with contracts enabled in order to work.