Elastic scaling: add e2e test (#3929)

On top of https://github.com/paritytech/polkadot-sdk/pull/3879

I've also moved the previous test where we ensure multiple cores per
para doesn't break non elastic parachains.

---------

Signed-off-by: Andrei Sandu <andrei-mihail@parity.io>
Co-authored-by: Javier Viola <363911+pepoviola@users.noreply.github.com>

polkadot/zombienet_tests/elastic_scaling/0001-basic-3cores-6s-blocks.toml

@@ -0,0 +1,42 @@
+[settings]
+timeout = 1000
+
+[relaychain.genesis.runtimeGenesis.patch.configuration.config.async_backing_params]
+  max_candidate_depth = 6
+  allowed_ancestry_len = 2
+
+[relaychain.genesis.runtimeGenesis.patch.configuration.config.scheduler_params]
+  max_validators_per_core = 1
+  scheduling_lookahead = 2
+  num_cores = 3
+
+[relaychain.genesis.runtimeGenesis.patch.configuration.config.approval_voting_params]
+  needed_approvals = 3
+  max_approval_coalesce_count = 5
+
+[relaychain]
+default_image = "{{ZOMBIENET_INTEGRATION_TEST_IMAGE}}"
+chain = "rococo-local"
+default_command = "polkadot"
+
+  [relaychain.default_resources]
+  limits = { memory = "4G", cpu = "2" }
+  requests = { memory = "2G", cpu = "1" }
+
+  [[relaychain.node_groups]]
+  name = "elastic-validator"
+  count = 5
+  args = [ "-lparachain=debug,parachain::candidate-backing=trace,parachain::provisioner=trace,parachain::prospective-parachains=trace,runtime=debug"]
+
+{% for id in range(2000,2002) %}
+[[parachains]]
+id = {{id}}
+addToGenesis = true
+
+    [parachains.collator]
+    name = "some-parachain"
+    image = "{{COL_IMAGE}}"
+    command = "adder-collator"
+    args = ["-lparachain::collation-generation=trace,parachain::collator-protocol=trace,parachain=debug"]
+{% endfor %}
+