Implemented test for existing clocks.

Implemented test for conversions between existing clocks
(sys_clock, utc_clock, tai_clock, gps_clock) showing that
they can be used instead of to_clock_time functions.

test/tz_test/clock_cast.pass.cpp

@@ -0,0 +1,102 @@
+// The MIT License (MIT)
+//
+// Copyright (c) 2017 Tomasz Kamiński
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+#include "tz.h"
+#include <cassert>
+
+int
+main()
+{
+    using namespace date;
+    using sys_clock = std::chrono::system_clock;
+
+    // self
+    {
+       sys_days st(1997_y/dec/12);
+       auto ut = utc_clock::from_sys(st);
+       auto tt = tai_clock::from_utc(ut);
+
+       assert(clock_cast<sys_clock>(st) == st);
+       assert(clock_cast<utc_clock>(ut) == ut);
+       assert(clock_cast<tai_clock>(tt) == tt);
+    }
+
+    // sys <-> utc
+    {
+       sys_days st(1997_y/dec/12);
+       auto ut = utc_clock::from_sys(st);
+
+       assert(clock_cast<utc_clock>(st) == ut);
+       assert(clock_cast<sys_clock>(ut) == st);      
+    }
+
+    // tai <-> utc
+    {
+       sys_days st(1997_y/dec/12);
+       auto ut = utc_clock::from_sys(st);
+       auto tt = tai_clock::from_utc(ut);
+
+       assert(clock_cast<tai_clock>(ut) == tt);
+       assert(clock_cast<utc_clock>(tt) == ut);
+    }
+
+    // tai <-> sys
+    {
+       sys_days st(1997_y/dec/12);
+       auto ut = utc_clock::from_sys(st);
+       auto tt = tai_clock::from_utc(ut);
+
+       assert(clock_cast<tai_clock>(st) == tt);
+       assert(clock_cast<sys_clock>(tt) == st);
+    }
+
+    // gps <-> utc
+    {
+       sys_days st(1997_y/dec/12);
+       auto ut = utc_clock::from_sys(st);
+       auto gt = gps_clock::from_utc(ut);
+
+       assert(clock_cast<gps_clock>(ut) == gt);
+       assert(clock_cast<utc_clock>(gt) == ut);
+    }
+
+    // gps <-> sys
+    {
+       sys_days st(1997_y/dec/12);
+       auto ut = utc_clock::from_sys(st);
+       auto gt = gps_clock::from_utc(ut);
+
+       assert(clock_cast<gps_clock>(st) == gt);
+       assert(clock_cast<sys_clock>(gt) == st);
+    }
+
+    // tai <-> gps   
+    {
+       sys_days st(1997_y/dec/12);
+       auto ut = utc_clock::from_sys(st);
+       auto tt = tai_clock::from_utc(ut);
+       auto gt = gps_clock::from_utc(ut);
+
+       assert(clock_cast<gps_clock>(tt) == gt);
+       assert(clock_cast<tai_clock>(gt) == tt);
+    }
+}