Add UI test for elif error message

testing/tests/ui/elif.rs

@@ -0,0 +1,8 @@
+use askama::Template;
+
+#[derive(Template)]
+#[template(source = "{% if true %}{% elif false %}{% endif %}", ext = "html")]
+struct UnknownElif;
+
+fn main() {
+}

testing/tests/ui/elif.stderr

@@ -0,0 +1,9 @@
+error: unknown `elif` keyword; did you mean `else if`?
+       problems parsing template source at row 1, column 16 near:
+       "elif false %}{% endif %}"
+ --> tests/ui/elif.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)