As of v0.18.1 I can use `test_bitcoin-qt -platform cocoa` to run GUI tests; this opens and closes a regtest GUI a bunch of times.

As of master (6e4f6553913978fcdddba30e0bb6f97edcb12e80) it seems to ignore the `-platform cocoa` and just run the test as with `minimal`.

As seen on macOS 10.14.6. Git bisect blames a2714a5c69f0b0506689af04c3e785f71ee0915d in #16578 `Do not pass in command line arguments to QApplication` (@achow101, @ryanofsky)
{
    QEventLoop loop;
    QTextEdit* messagesWidget = console->findChild<QTextEdit*>("messagesWidget");
    QObject::connect(messagesWidget, &QTextEdit::textChanged, &loop, &QEventLoop::quit);
    QLineEdit* lineEdit = console->findChild<QLineEdit*>("lineEdit");
    QTest::keyClicks(lineEdit, "getblockchaininfo");
    QTest::keyClick(lineEdit, Qt::Key_Return);
    loop.exec();
    QString output = messagesWidget->toPlainText();
    UniValue value;
    value.read(output.right(output.size() - output.lastIndexOf(QChar::ObjectReplacementCharacter) - 1).toStdString());
    QCOMPARE(value["chain"].get_str(), std::string("regtest"));
}
} 
void AppTests::appTests()