js: add basic support for running tests, fix string.replace_each (#11314)

vlib/builtin/js/builtin.js.v

@@ -36,3 +36,11 @@ pub fn eprint(s any) {
 		panic('Cannot `eprint` in a browser, use `println` instead')
 	}
 }
+
+// Exits the process in node, and halts execution in the browser
+// because `process.exit` is undefined. Workaround for not having
+// a 'real' way to exit in the browser.
+pub fn exit(c int) {
+	JS.process.exit(c)
+	js_throw('exit($c)')
+}