Add simple test infrastructure for JS DCE

Add test infrastructure that allows to test handwritten JS
to test various small aspects of DCE, as opposed to box tests
which test kotlin stdlib + kotlin generated examples

js/js.translator/testData/dce/commonjs.js

@@ -0,0 +1,17 @@
+function foo() {
+}
+function bar() {
+}
+function ignore() {
+}
+
+bar();
+
+module.exports.foo = foo;
+module.exports.bar = bar;
+module.exports.ignore = ignore;
+
+// REQUEST_REACHABLE: main.foo
+// ASSERT_REACHABLE: main.bar
+// ASSERT_REACHABLE: main.foo
+// ASSERT_UNREACHABLE: main.ignore