LibJS/Tests: Use eval() for toEvalTo(), not Function()

Since we have had eval() for a while now, we can finally use it here -
this allows us to get rid of the confusing return statements in tested
source code.

Userland/Libraries/LibJS/Tests/test-common.js

@@ -354,7 +354,7 @@ class ExpectationError extends Error {
             let result;
 
             try {
-                result = new Function(this.target)();
+                result = eval(this.target);
             } catch (e) {
                 throw new ExpectationError();
             }