test: add interface_gui.py to test bitcoin-gui startup via RPC

Adds a functional test that starts bitcoin-gui (via the bitcoin wrapper
with "gui" subcommand) using QT_QPA_PLATFORM=minimal for headless
operation, then verifies it responds to a stop RPC call. This catches
startup regressions in the GUI that have no CI coverage today.

Extends the test framework to support use_gui=True in TestNode, which
invokes "bitcoin gui" instead of "bitcoin node" and automatically sets
QT_QPA_PLATFORM=minimal. Adds BUILD_GUI to test/config.ini.in and
is_gui_compiled()/skip_if_no_gui() helpers to BitcoinTestFramework.

Co-Authored-By: MarcoFalke <*~=`'#}+{/-|&$^_@721217.xyz>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/functional/test_framework/test_framework.py

@@ -1078,6 +1078,11 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         if not self.is_ipc_compiled():
             raise SkipTest("ipc has not been compiled.")
 
+    def skip_if_no_gui(self):
+        """Skip the running test if the GUI has not been compiled."""
+        if not self.is_gui_compiled():
+            raise SkipTest("GUI has not been compiled.")
+
     def skip_if_no_previous_releases(self):
         """Skip the running test if previous releases are not available."""
         if not self.has_previous_releases():
@@ -1160,6 +1165,10 @@ class BitcoinTestFramework(metaclass=BitcoinTestMetaClass):
         """Checks whether ipc was compiled."""
         return self.config["components"].getboolean("ENABLE_IPC")
 
+    def is_gui_compiled(self):
+        """Checks whether the GUI was compiled."""
+        return self.config["components"].getboolean("BUILD_GUI", fallback=False)
+
     def has_blockfile(self, node, filenum: str):
         return (node.blocks_path/ f"blk{filenum}.dat").is_file()