Add ui test for filter recursion

Author: djc

testing/tests/ui/filter-recursion.rs

@@ -0,0 +1,9 @@
+use askama::Template;
+
+#[derive(Template)]
+#[template(source = "{{ s|a|a|a|a|a|a|a|A|a|a|a|a|a|a|a|a|a|a|a|a|a", ext = "txt")]
+struct Filtered {
+    s: &'static str,
+}
+
+fn main() {}

testing/tests/ui/filter-recursion.stderr

@@ -0,0 +1,8 @@
+error: problems parsing template source at row 1, column 46 near:
+       ""
+ --> tests/ui/filter-recursion.rs:3:10
+  |
+3 | #[derive(Template)]
+  |          ^^^^^^^^
+  |
+  = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)