Make wxTimeSpan::operator-() const

Due to an oversight, it wasn't declared as const, making it impossible to
subtract from a const wxTimeSpan object.

Fix this and add a unit test verifying that this compiles and works as
expected.

Closes #17583.

include/wx/datetime.h

@@ -1210,7 +1210,7 @@ public:
     inline wxTimeSpan& Subtract(const wxTimeSpan& diff);
         // subtract another timespan
     wxTimeSpan& operator-=(const wxTimeSpan& diff) { return Subtract(diff); }
-    inline wxTimeSpan operator-(const wxTimeSpan& ts)
+    inline wxTimeSpan operator-(const wxTimeSpan& ts) const
     {
         return wxTimeSpan(GetValue() - ts.GetValue());
     }

tests/datetime/datetimetest.cpp

@@ -1346,6 +1346,10 @@ void DateTimeTestCase::TestTimeArithmetics()
 
     // And a reverse. Now we should use days in Jun (again 30 => 4w 1d)
     CPPUNIT_ASSERT_EQUAL( wxDateSpan(0, -10, -4, -1), dtd1.DiffAsDateSpan(dtd2) );
+
+    const wxTimeSpan ts1 = wxTimeSpan::Seconds(30);
+    const wxTimeSpan ts2 = wxTimeSpan::Seconds(5);
+    CPPUNIT_ASSERT_EQUAL( wxTimeSpan::Seconds(25), ts1 - ts2 );
 }
 
 void DateTimeTestCase::TestDSTBug()

tests/testdate.h

@@ -30,6 +30,17 @@ inline std::ostream& operator<<(std::ostream& ostr, const wxDateSpan& span)
     return ostr;
 }
 
+inline std::ostream& operator<<(std::ostream& ostr, const wxTimeSpan& span)
+{
+    ostr << span.GetWeeks() << "W, "
+         << span.GetDays() << "D, "
+         << span.GetHours() << ":"
+         << span.GetMinutes() << ":"
+         << span.GetSeconds() << "."
+         << span.GetMilliseconds();
+
+    return ostr;
+}
 WX_CPPUNIT_ALLOW_EQUALS_TO_INT(wxDateTime::wxDateTime_t)
 
 #endif // _WX_TESTS_TESTDATE_H_