Add test for fluent diagnostics

Author: Xiretza

Diff for: tests/ui-fulldeps/fluent-messages/many-lines.ftl

@@ -0,0 +1,11 @@
+no_crate_foo = foo
+
+# This file tests error reporting for
+# fluent files with many lines.
+# The error message should point to the correct line number
+# and include no more context than necessary.
+
+no_crate_bar =
+
+no_crate_baz =
+    baz

Diff for: tests/ui-fulldeps/fluent-messages/test.rs

@@ -80,3 +80,8 @@ mod bad_escape {
     //~| ERROR invalid escape `\"`
     //~| ERROR invalid escape `\'`
 }
+
+mod many_lines {
+    rustc_fluent_macro::fluent_messages! { "./many-lines.ftl" }
+    //~^ ERROR could not parse Fluent resource
+}

Diff for: tests/ui-fulldeps/fluent-messages/test.stderr

@@ -103,5 +103,20 @@ LL |     rustc_fluent_macro::fluent_messages! { "./invalid-escape.ftl" }
    |
    = note: Fluent does not interpret these escape sequences (<https://projectfluent.org/fluent/guide/special.html>)
 
-error: aborting due to 13 previous errors
+error: could not parse Fluent resource
+  --> $DIR/test.rs:85:44
+   |
+LL |     rustc_fluent_macro::fluent_messages! { "./many-lines.ftl" }
+   |                                            ^^^^^^^^^^^^^^^^^^
+   |
+   = help: see additional errors emitted
+
+error: expected a message field for "no_crate_bar"
+  --> ./many-lines.ftl:15:1
+   |
+15 | no_crate_bar =
+   | ^^^^^^^^^^^^^^
+   |
+
+error: aborting due to 14 previous errors