docs: add state_key in run block documentation

Author: kevineor

website/docs/language/tests/index.mdx

@@ -91,6 +91,7 @@ Each `run` block has the following fields and blocks:
 | [`providers`](#providers) | An optional `providers` attribute.                                                                                       |               |
 | [`assert`](#assertions)   | Optional `assert` blocks.                                                                                                |               |
 | `expect_failures`         | An optional attribute.                                                                                                   |               |
+| `state_key`               | An optional attribute, which can be used to test multiple run blocks on the same infrastructure                          |               |
 
 The `command` attribute and `plan_options` block tell Terraform which command and options to execute for each run block. The default operation, if you do not specify a `command` attribute or the `plan_options` block, is a normal Terraform apply operation.