test framework: add support for testing files to fmt command (#33576)

internal/command/testdata/tftest-fmt/main_out.tftest.hcl

@@ -0,0 +1,18 @@
+
+variables {
+  first  = "value"
+  second = "value"
+}
+
+run "some_run_block" {
+  command = plan
+
+  plan_options = {
+    refresh = false
+  }
+
+  assert {
+    condition     = var.input == 12
+    error_message = "something"
+  }
+}