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: nodejs#11144
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

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'),
+                   'Bücher@xn--wgv71a119e.com');
+assert.strictEqual(punycode.toUnicode('Bücher@xn--wgv71a119e.com'),
+                   'Bücher@日本語.com');