qa: Enable interface_gui.py on macOS

This is a follow-up to bitcoin/bitcoin#35551.

test/functional/interface_gui.py

@@ -21,11 +21,9 @@ class GuiTest(BitcoinTestFramework):
         self.skip_if_no_gui()
         # On Windows, bitcoin.exe exits immediately when launching bitcoin-gui.exe,
         # causing the test framework's process monitor to see a premature node exit.
-        # On macOS, bitcoin-qt's Cocoa code assumes NSApp is initialized, but the
-        # minimal Qt platform plugin skips that, causing crashes.
-        # Both issues are likely fixable.
-        if platform.system() in ("Windows", "Darwin"):
-            raise SkipTest("bitcoin-gui test not supported on Windows or macOS")
+        # This issue is likely fixable.
+        if platform.system() == "Windows":
+            raise SkipTest("bitcoin-gui test not supported on Windows")
 
     def setup_nodes(self):
         self.extra_init = [{"use_gui": True}]

test/functional/test_framework/test_node.py

@@ -282,6 +282,11 @@ class TestNode():
         subp_env = dict(os.environ, LIBC_FATAL_STDERR_="1")
         if self.use_gui:
             subp_env.setdefault("QT_QPA_PLATFORM", "minimal")
+            if platform.system() == "Darwin":
+                # QMacStyle assumes a Cocoa platform window, which the minimal platform
+                # does not provide. In particular, painting a QGroupBox can make Qt call
+                # addSubview: on an invalid native object (QTBUG-49686).
+                subp_env.setdefault("QT_STYLE_OVERRIDE", "fusion")
             subp_env.setdefault("LC_ALL", "nl_NL.UTF-8") # Set language to try to trigger translation bugs
             if sys.platform.startswith("linux") and "XDG_RUNTIME_DIR" not in subp_env:
                 # Qt prints warnings to stderr when XDG_RUNTIME_DIR is unset or has wrong