Fix displaying wxSpinCtrlDouble values with default precision

For the sake of consistency with native wxGTK implementation,
spin control value should be displayed with fixed precision.

See #17085.

include/wx/generic/spinctlg.h

@@ -420,7 +420,7 @@ private:
     void Init()
     {
         m_digits = 0;
-        m_format = wxS("%g");
+        m_format = "%0.0f";
     }
 
     wxString m_format;

tests/controls/spinctrldbltest.cpp

@@ -199,7 +199,7 @@ void SpinCtrlDoubleTestCase::Increment()
 #if wxUSE_UIACTIONSIMULATOR
     CPPUNIT_ASSERT_EQUAL(1.0, m_spin->GetIncrement());
 
-    m_spin->SetDigits(1); // GTK would fail without this.
+    m_spin->SetDigits(1);
     m_spin->SetIncrement(0.1);
 
     CPPUNIT_ASSERT_EQUAL(0.1, m_spin->GetIncrement());