Add test for invalid variables

tests/ui-fulldeps/session-diagnostic/example.ftl

@@ -3,3 +3,5 @@ no_crate_example = this is an example message used in testing
     .help = with a help
     .suggestion = with a suggestion
     .label = with a label
+
+no_crate_bad_reference = {$r} does not exist

tests/ui-fulldeps/session-diagnostic/invalid-variable.rs

@@ -0,0 +1,21 @@
+// run-fail
+// compile-flags: --test
+// test that messages referencing non-existent fields cause test failures
+
+#![feature(rustc_private)]
+#![crate_type = "lib"]
+
+extern crate rustc_driver;
+extern crate rustc_fluent_macro;
+extern crate rustc_macros;
+extern crate rustc_errors;
+use rustc_fluent_macro::fluent_messages;
+use rustc_macros::Diagnostic;
+use rustc_errors::{SubdiagnosticMessage, DiagnosticMessage};
+extern crate rustc_session;
+
+fluent_messages! { "./example.ftl" }
+
+#[derive(Diagnostic)]
+#[diag(no_crate_bad_reference)]
+struct BadRef;