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>
