Add a FluentAssertions approval failure reporter

So that we have a clear error to investigate in case `ApprovePublicApi` fails on the continuous integration server.

Author: 0xced

tests/PublicApi.cs

@@ -1,8 +1,10 @@
 using System.IO;
 using System.Reflection;
 using ApprovalTests;
+using ApprovalTests.Core;
 using ApprovalTests.Namers;
 using ApprovalTests.Reporters;
+using FluentAssertions;
 using PublicApiGenerator;
 using Xunit;
 
@@ -16,7 +18,7 @@ public void ApprovePublicApi()
             var assembly = typeof(Log4NetTextFormatter).Assembly;
             var publicApi = assembly.GeneratePublicApi();
             var writer = new ApprovalTextWriter(publicApi);
-            Approvals.Verify(writer, new AssemblyNamer(assembly), new DiffReporter());
+            Approvals.Verify(writer, new AssemblyNamer(assembly), new MultiReporter(DiffReporter.INSTANCE, new FluentAssertionReporter()));
         }
 
         private class AssemblyNamer : UnitTestFrameworkNamer
@@ -27,5 +29,12 @@ private class AssemblyNamer : UnitTestFrameworkNamer
 
             public override string Name => nameof(PublicApi) + "." + Path.GetFileNameWithoutExtension(_assembly.Location);
         }
+
+        // So that we have the full approved + received text written on the console in case it fails on continuous integration server
+        // where the DiffEngine used by `DiffReporter` is [automatically disabled](https://github.com/VerifyTests/DiffEngine/blob/6.4.5/src/DiffEngine/DisabledChecker.cs#L10).
+        private class FluentAssertionReporter : IApprovalFailureReporter
+        {
+            public void Report(string approved, string received) => File.ReadAllText(received).Should().Be(File.ReadAllText(approved));
+        }
     }
 }