LibJS: Restrict toEval() failures to SyntaxError

We only use expect(...).toEval() / not.toEval() for checking syntax
errors, where we obviously can't put the code in a regular function. For
runtime errors we do exactly that, so toEval() should not fail - this
allows us to use undefined identifiers in syntax tests.

Libraries/LibJS/Tests/test-common.js

@@ -287,7 +287,7 @@ class ExpectationError extends Error {
 
             let threw = false;
             try {
-                new Function(this.target)();
+                new Function(this.target);
             } catch (e) {
                 threw = true;
             }