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)
#include <qt/bitcoin.h>
#include <qt/test/apptests.h>
#include <qt/test/rpcnestedtests.h>
#include <qt/test/uritests.h>
#include <qt/test/compattests.h>
#include <test/setup_common.h>
#ifdef ENABLE_WALLET
#include <qt/test/addressbooktests.h>
#ifdef ENABLE_BIP70
#include <qt/test/paymentservertests.h>
#endif 
#include <qt/test/wallettests.h>
#endif 
#include <QApplication>
#include <QObject>
#include <QTest>
#if defined(QT_STATICPLUGIN)
#include <QtPlugin>
#if defined(QT_QPA_PLATFORM_MINIMAL)
Q_IMPORT_PLUGIN(QMinimalIntegrationPlugin);
#endif
#if defined(QT_QPA_PLATFORM_XCB)
Q_IMPORT_PLUGIN(QXcbIntegrationPlugin);
#elif defined(QT_QPA_PLATFORM_WINDOWS)
Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin);
#elif defined(QT_QPA_PLATFORM_COCOA)
Q_IMPORT_PLUGIN(QCocoaIntegrationPlugin);
#endif
#endif
int main(int argc, char *argv[])