fix CppunitTest_dbaccess_SingleSelectQueryComposer_test deadlock

I have seen several deadlocks on jenkins in this unit test, normally
with stacks like

 privateSnippetExecutor ()
 connectivity::hsqldb::OHsqlConnection::flush()
 connectivity::hsqldb::ODriverDelegator::flushConnections()
 connectivity::hsqldb::OConnectionController::queryTermination
 framework::Desktop::impl_sendQueryTerminationEvent
 framework::Desktop::terminate()
(anonymous namespace)::Hook::deinitHook((anonymous namespace)::Hook*,
LinkParamNone*)
 (anonymous namespace)::Hook::LinkStubdeinitHook(void*, LinkParamNone*)
 Link<LinkParamNone*, void>::Call(LinkParamNone*) const
 in DeInitVCL()

Implement a proper teardown in the unit test so this does not happen.

Change-Id: I3242752dc38d74128fcd134fe7391f04bd64ebc7
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/183391
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

dbaccess/qa/unit/SingleSelectQueryComposer_test.cxx

@@ -41,6 +41,7 @@ class SingleSelectQueryComposerTest : public DBTestBase
 
 public:
     virtual void setUp() override;
+    virtual void tearDown() override;
     void testSetCommand();
     void testAttributes();
     void testSubQueries();
@@ -83,6 +84,17 @@ void SingleSelectQueryComposerTest::setUp()
     m_xComposer = xComposer;
 }
 
+void SingleSelectQueryComposerTest::tearDown()
+{
+    m_xComposer.clear();
+    if (m_xConnection)
+    {
+        m_xConnection->close();
+        m_xConnection.clear();
+    }
+    DBTestBase::tearDown();
+}
+
 // tests setCommand of the composer
 
 void SingleSelectQueryComposerTest::testSetCommand()