Introduces IProcessImpl.Exit and uses it

Adds IProcessImpl.Exit for controlled process termination, replaces Environment.Exit with IProcessImpl.Exit calls, and adds a test implementation for IProcessImpl.

src/lib-csharp/Locators/DefaultProcessImpl.cs

@@ -75,5 +75,7 @@ namespace Velopack.Locators
                 _logger.Error($"Process {p.Id} has already exited.");
             }
         }
+
+        public void Exit(int exitCode) => Environment.Exit(exitCode);
     }
 }

src/lib-csharp/Locators/IProcessImpl.cs

@@ -9,5 +9,11 @@ namespace Velopack.Locators
         string GetCurrentProcessPath();
         uint GetCurrentProcessId();
         void StartProcess(string exePath, IEnumerable<string> args, string workDir, bool showWindow);
+
+        /// <summary>
+        /// Exit the current process with the given exit code.
+        /// </summary>
+        /// <param name="exitCode">The exit code</param>
+        void Exit(int exitCode);
     }
 }

src/lib-csharp/Locators/TestVelopackLocator.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
 using NuGet.Versioning;
@@ -86,6 +86,8 @@ namespace Velopack.Locators
 
         public override IProcessImpl Process => this;
 
+        public int? ExitCode { get; private set; }
+
         public override VelopackAsset? GetLatestLocalFullPackage()
         {
             if (_asset != null) {
@@ -142,5 +144,13 @@ namespace Velopack.Locators
         {
             new DefaultProcessImpl(Log).StartProcess(exePath, args, workDir, showWindow);
         }
+
+        public void Exit(int exitCode)
+        {
+            if (ExitCode is not null) {
+                throw new InvalidOperationException("Exit has already been called on this instance.");
+            }
+            ExitCode = exitCode;
+        }
     }
 }

src/lib-csharp/UpdateManager.Helpers.cs

@@ -16,7 +16,7 @@ namespace Velopack
         public void ApplyUpdatesAndRestart(VelopackAsset? toApply, string[]? restartArgs = null)
         {
             WaitExitThenApplyUpdates(toApply, silent: false, restart: true, restartArgs);
-            Environment.Exit(0);
+            Locator.Process.Exit(0);
         }
 
         /// <summary>
@@ -29,7 +29,7 @@ namespace Velopack
         public void ApplyUpdatesAndExit(VelopackAsset? toApply)
         {
             WaitExitThenApplyUpdates(toApply, silent: true, restart: false);
-            Environment.Exit(0);
+            Locator.Process.Exit(0);
         }
         
         /// <summary>

src/lib-csharp/VelopackApp.cs

@@ -276,9 +276,7 @@ namespace Velopack
 
         private static void Exit(int code)
         {
-            if (!VelopackRuntimeInfo.InUnitTestRunner) {
-                Environment.Exit(code);
-            }
+            VelopackLocator.GetCurrentOrCreateDefault().Process.Exit(code);
         }
     }
 }