Run bitcoin_test-qt under minimal QPA platform

Fixes broken "make check" reported by Matt Corallo <git@bluematt.me> in
https://github.com/bitcoin/bitcoin/issues/10110

Fix was suggested and initially implemented by
Cory Fields <cory-nospam-@coryfields.com> in
https://github.com/bitcoin/bitcoin/pull/10117#issuecomment-290275236

build-aux/m4/bitcoin_qt.m4

@@ -130,6 +130,8 @@ AC_DEFUN([BITCOIN_QT_CONFIGURE],[
       if test "x$bitcoin_cv_need_acc_widget" = "xyes"; then
         _BITCOIN_QT_CHECK_STATIC_PLUGINS([Q_IMPORT_PLUGIN(AccessibleFactory)], [-lqtaccessiblewidgets])
       fi
+      _BITCOIN_QT_CHECK_STATIC_PLUGINS([Q_IMPORT_PLUGIN(QMinimalIntegrationPlugin)],[-lqminimal])
+      AC_DEFINE(QT_QPA_PLATFORM_MINIMAL, 1, [Define this symbol if the minimal qt platform exists])
       if test x$TARGET_OS = xwindows; then
         _BITCOIN_QT_CHECK_STATIC_PLUGINS([Q_IMPORT_PLUGIN(QWindowsIntegrationPlugin)],[-lqwindows])
         AC_DEFINE(QT_QPA_PLATFORM_WINDOWS, 1, [Define this symbol if the qt platform is windows])

src/qt/test/test_main.cpp

@@ -31,6 +31,9 @@ Q_IMPORT_PLUGIN(qjpcodecs)
 Q_IMPORT_PLUGIN(qtwcodecs)
 Q_IMPORT_PLUGIN(qkrcodecs)
 #else
+#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)
@@ -53,6 +56,11 @@ int main(int argc, char *argv[])
 
     bool fInvalid = false;
 
+    // Prefer the "minimal" platform for the test instead of the normal default
+    // platform ("xcb", "windows", or "cocoa") so tests can't unintentially
+    // interfere with any background GUIs and don't require extra resources.
+    setenv("QT_QPA_PLATFORM", "minimal", 0);
+
     // Don't remove this, it's needed to access
     // QApplication:: and QCoreApplication:: in the tests
     QApplication app(argc, argv);

src/qt/test/wallettests.cpp

@@ -68,6 +68,18 @@ QModelIndex FindTx(const QAbstractItemModel& model, const uint256& txid)
 }
 
 //! Simple qt wallet tests.
+//
+// Test widgets can be debugged interactively calling show() on them and
+// manually running the event loop, e.g.:
+//
+//     sendCoinsDialog.show();
+//     QEventLoop().exec();
+//
+// This also requires overriding the default minimal Qt platform:
+//
+//     src/qt/test/test_bitcoin-qt -platform xcb      # Linux
+//     src/qt/test/test_bitcoin-qt -platform windows  # Windows
+//     src/qt/test/test_bitcoin-qt -platform cocoa    # macOS
 void WalletTests::walletTests()
 {
     // Set up wallet and chain with 101 blocks (1 mature block for spending).