test: add TZ mocker and tests

Author: anthonyreilly

src/NetCoreForce.Client.Tests/TimeZoneIds.cs

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+using System;
+using System.Reflection;
+
+namespace NetCoreForce.Client.Tests
+{   
+    /// <summary>
+    /// Sample TimeZone IDs for use in tests
+    /// </summary>
+    public static class TimeZoneIds
+    {
+        public static string TimeZoneIdNewYork = "America/New_York";
+        public static string TimeZoneIdPhoenix = "America/Phoenix";
+        public static string TimeZoneIdLondon = "Europe/London";
+        public static string TimeZoneIdTokyo = "Asia/Tokyo";
+    }
+}

src/NetCoreForce.Client.Tests/TimeZoneInfoMocker.cs

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+using System;
+using System.Reflection;
+
+namespace NetCoreForce.Client.Tests
+{
+    public class LocalTimeZoneInfoMocker : IDisposable
+    {
+        public LocalTimeZoneInfoMocker(TimeZoneInfo mockTimeZoneInfo)
+        {
+            var info = typeof(TimeZoneInfo).GetField(
+                "s_cachedData",
+                BindingFlags.NonPublic | BindingFlags.Static
+                );
+
+            var cachedData = info.GetValue(null);
+
+            var field = cachedData.GetType().GetField(
+                "_localTimeZone",
+                BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static | BindingFlags.Instance
+                );
+
+            field.SetValue(cachedData, mockTimeZoneInfo);
+        }
+
+        public void Dispose()
+        {
+            TimeZoneInfo.ClearCachedData();
+        }
+    }
+}

src/NetCoreForce.Client.Tests/TimeZoneInfoMockerTests.cs

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+using System;
+using System.Collections.Generic;
+using Xunit;
+using NetCoreForce.Client;
+using NetCoreForce.Client.Models;
+
+namespace NetCoreForce.Client.Tests
+{
+    public class TimeZoneInfoMockerTests
+    {
+        [Fact]
+        public void TestLocalTimeZoneInfoMocker()
+        {
+            TimeSpan mockedUtcOffset;
+            TimeSpan actualLocalUtcOffset = TimeZoneInfo.Local.BaseUtcOffset;
+
+            using (new LocalTimeZoneInfoMocker(TimeZoneInfo.FindSystemTimeZoneById(TimeZoneIds.TimeZoneIdPhoenix)))
+            {
+                // shifted to Phoenix TZ
+                Assert.Equal(TimeZoneIds.TimeZoneIdPhoenix, TimeZoneInfo.Local.Id);
+                mockedUtcOffset = TimeZoneInfo.Local.BaseUtcOffset;
+                Assert.NotEqual(TimeZoneInfo.Local.BaseUtcOffset, actualLocalUtcOffset);
+            }
+
+            // back to local machine's TZ
+            Assert.NotEqual(TimeZoneInfo.Local.Id, TimeZoneIds.TimeZoneIdPhoenix);
+            Assert.NotEqual(TimeZoneInfo.Local.BaseUtcOffset, mockedUtcOffset);
+        }
+
+    }
+}