JS: add test case for arrow functions with duplicate parameter names

Author: esbena

javascript/ql/test/query-tests/Declarations/UniqueParameterNames/tst.js

@@ -17,3 +17,5 @@ function f(
 x,
 x // OK: empty function
 ) { }
+
+(a, a) => a + a; // OK: for strict mode functions, duplicate parameter names are a syntax error

javascript/ql/test/query-tests/LanguageFeatures/SyntaxError/SyntaxError.expected

@@ -1 +1,2 @@
+| arrows.js:1:5:1:5 | Error: Argument name clash | Error: Argument name clash |
 | tst.js:2:12:2:12 | Error: Unterminated string constant | Error: Unterminated string constant |

javascript/ql/test/query-tests/LanguageFeatures/SyntaxError/arrows.js

@@ -0,0 +1 @@
+(a, a) => a + a;