autotests: fix the failures I got here.

1) some of them can't run in parallel
2) kconfigskeletontest failed due to KSharedConfig being used under the
scene (for a year now), and there's probably some QPA or widget style
holding a ref (here, not in the CI). Indirectly, simply setting
QStandardPaths::setTestModeEnabled(true) fixes this since KSharedConfig
will then not share the instance with the non-test-mode-enabled instance.

autotests/CMakeLists.txt

@@ -61,5 +61,10 @@ ecm_add_tests(
    LINK_LIBRARIES KF5::ConfigGui Qt5::Test
 )
 
+# These tests do a global cleanup of ~/.qttest, so they can't run in parallel
+set_tests_properties(kconfigcore-kconfigtest PROPERTIES RUN_SERIAL TRUE)
+set_tests_properties(kconfigcore-kconfignokdehometest PROPERTIES RUN_SERIAL TRUE)
+set_tests_properties(kconfiggui-kconfigguitest PROPERTIES RUN_SERIAL TRUE)
+
 add_subdirectory(kconfig_compiler)
 

autotests/kconfigskeletontest.cpp

@@ -35,6 +35,11 @@ QTEST_MAIN(KConfigSkeletonTest)
 #define WRITE_SETTING3 QFont("helvetica",14)
 #define WRITE_SETTING4 QString("KDE")
 
+void KConfigSkeletonTest::initTestCase()
+{
+    QStandardPaths::setTestModeEnabled(true);
+}
+
 void KConfigSkeletonTest::init()
 {
     QFile::remove(QStandardPaths::writableLocation(QStandardPaths::GenericConfigLocation) + "/kconfigskeletontestrc");

autotests/kconfigskeletontest.h

@@ -27,6 +27,7 @@ class KConfigSkeletonTest : public QObject
 public:
 
 private Q_SLOTS:
+    void initTestCase();
     void init();
     void cleanup();
     void testSimple();