test: improve punycode test coverage

Adds two additional tests for mapDomain function in punycode.js.
When an email address is given to the toASCII() and toUnicode()
functions, only the parts before the '@' character should be
encoded/decoded.

PR-URL: #11144
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>

Author: seppevs

test/parallel/test-punycode.js

@@ -237,3 +237,9 @@ assert.strictEqual(punycode.ucs2.encode([0xDC00]), '\uDC00');
 assert.strictEqual(punycode.ucs2.encode([0xDC00, 0x61, 0x62]), '\uDC00ab');
 
 assert.strictEqual(errors, 0);
+
+// test map domain
+assert.strictEqual(punycode.toASCII('Bücher@日本語.com'),
+                   'Bü[email protected]');
+assert.strictEqual(punycode.toUnicode('Bü[email protected]'),
+                   'Bücher@日本語.com');