Don't decrease the number of digits implicitly

wxSpinCtrlDouble::SetIncrement() should increase the number of digits if
necessary because not doing it would be inconsistent with the initial
determination of the number of digits in the ctor and would actually
lose the digits displayed, but it seems unnecessary to decrease the
number of digits and it might be surprising, so don't do it.

Add a test for this behaviour and document it.

include/wx/generic/spinctlg.h

@@ -384,7 +384,7 @@ public:
                 double inc = 1,
                 const wxString& name = wxT("wxSpinCtrlDouble"))
     {
-        DetermineDigits(inc);
+        DoSetDigits(DetermineDigits(inc));
         return wxSpinCtrlGenericBase::Create(parent, id, value, pos, size,
                                              style, min, max, initial,
                                              inc, name);
@@ -426,8 +426,9 @@ private:
         DoSetDigits(0);
     }
 
-    // Update m_digits and m_format to correspond to the given increment.
-    void DetermineDigits(double inc);
+    // Return the number of digits required to show the numbers using the
+    // specified increment without loss of precision.
+    static unsigned DetermineDigits(double inc);
 
     // Set the number of digits and the format unconditionally.
     void DoSetDigits(unsigned digits);