testing framework: change test file extension to .tftest.hcl (#33553)

internal/command/testdata/test/with_provider_alias/main.tftest.hcl

@@ -0,0 +1,37 @@
+provider "test" {
+  data_prefix = "data"
+  resource_prefix = "resource"
+}
+
+provider "test" {
+  alias = "setup"
+
+  # The setup provider will write into the main providers data sources.
+  resource_prefix = "data"
+}
+
+variables {
+  managed_id = "B853C121"
+}
+
+run "setup" {
+  module {
+    source = "./setup"
+  }
+
+  variables {
+    value = "Hello, world!"
+    id = "B853C121"
+  }
+
+  providers = {
+    test = test.setup
+  }
+}
+
+run "test" {
+  assert {
+    condition = test_resource.created.value == "Hello, world!"
+    error_message = "bad value"
+  }
+}