Add support for custom qtbase rev

scripts/checkout-qt.sh

@@ -8,6 +8,12 @@ git fetch --depth 1 origin "$qt5_rev"
 git checkout FETCH_HEAD
 git submodule update --init -- qtbase qtxmlpatterns qtdeclarative qtwayland
 
+if [[ -v QT_DOCKERTEST_QTBASE_REV ]] ; then
+    cd "$srcdir/qtbase"
+    git fetch --depth 1 origin "$QT_DOCKERTEST_QTBASE_REV"
+    git checkout FETCH_HEAD
+fi
+
 if [[ -v QT_DOCKERTEST_QTWAYLAND_REV ]] ; then
     cd "$srcdir/qtwayland"
     git fetch --depth 1 origin "$QT_DOCKERTEST_QTWAYLAND_REV"