Add ui test for extends error

testing/tests/ui/extends.rs

@@ -0,0 +1,18 @@
+use askama::Template;
+
+#[derive(Template)]
+#[template(
+    source = r#"{%- extends "whatever.html" %}"#,
+    ext = "html"
+)]
+struct ExtendsPre;
+
+#[derive(Template)]
+#[template(
+    source = r#"{% extends "whatever.html" -%}"#,
+    ext = "html"
+)]
+struct ExtendsPost;
+
+fn main() {
+}

testing/tests/ui/extends.stderr

@@ -0,0 +1,19 @@
+error: whitespace control is not allowed on `extends`
+       problems parsing template source at row 1, column 2 near:
+       "- extends \"whatever.html\" %}"
+ --> tests/ui/extends.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)
+
+error: whitespace control is not allowed on `extends`
+       problems parsing template source at row 1, column 2 near:
+       " extends \"whatever.html\" -%}"
+  --> tests/ui/extends.rs:10:10
+   |
+10 | #[derive(Template)]
+   |          ^^^^^^^^
+   |
+   = note: this error originates in the derive macro `Template` (in Nightly builds, run with -Z macro-backtrace for more info)