Utils: Turn "Util.asciify" into a plain macro

This monves the asciify function to stringutils and makes it directly
available as "asciify:" prefix macro, so that the generation of a
default build path does not go through JavaScript.

"Util.asciify" remains available as core JavaScript extension for the
case that it is used by third party code/wizards.

This change also adds a test to tst_stringutils

Change-Id: Iba2f20c0415ee8fe757c2f0058a90629b3fbeff0
Reviewed-by: hjk <hjk@qt.io>

src/plugins/coreplugin/corejsextensions.cpp

@@ -139,14 +139,7 @@ QString UtilsJsExtension::mktemp(const QString &pattern) const
 
 QString UtilsJsExtension::asciify(const QString &input) const
 {
-    QString result;
-    for (const QChar &c : input) {
-        if (c.isPrint() && c.unicode() < 128)
-            result.append(c);
-        else
-            result.append(QString::fromLatin1("u%1").arg(c.unicode(), 4, 16, QChar('0')));
-    }
-    return result;
+    return Utils::asciify(input);
 }
 
 QString UtilsJsExtension::qtQuickVersion(const QString &filePath) const