add @ error tests

tests/at-error.rs

@@ -0,0 +1,15 @@
+#![cfg(test)]
+
+#[macro_use]
+mod macros;
+
+error!(
+    error_dblquoted_string,
+    "a {\n  @error \"hi\";\n}\n", "Error: \"hi\""
+);
+error!(
+    error_sglquoted_string,
+    "a {\n  @error 'hi';\n}\n", "Error: \"hi\""
+);
+error!(error_unquoted_string, "a {\n  @error hi;\n}\n", "Error: hi");
+error!(error_arithmetic, "a {\n  @error 1 + 1;\n}\n", "Error: 2");