Add a test for well-known source-branches

These two tests work together to prove success and failure scenarios work as expected.

Author: michaelnoonan

src/GitVersionCore.Tests/Configuration/ConfigProviderTests.cs

@@ -117,8 +117,8 @@ public void SourceBranchIsRequired()
                                 "See https://gitversion.net/docs/configuration/ for more info");
         }
 
-        [Test]
-        public void SourceBranchesShouldExist()
+        [Test(Description = "This test proves the configuration validation will fail early with a helpful message when a branch listed in source-branches has no configuration.")]
+        public void SourceBranchesValidationShouldFailWhenMatchingBranchConfigurationIsMissing()
         {
             const string text = @"
 branches:
@@ -132,6 +132,23 @@ public void SourceBranchesShouldExist()
                                 "See https://gitversion.net/docs/configuration/ for more info");
         }
 
+        [Test(Description = "Well-known branches may not be present in the configuration file. This test confirms the validation check succeeds when the source-branches configuration contain these well-known branches.")]
+        [TestCase(Config.MasterBranchKey)]
+        [TestCase(Config.DevelopBranchKey)]
+        public void SourceBranchesValidationShouldSucceedForWellKnownBranches(string wellKnownBranchKey)
+        {
+            var text = $@"
+branches:
+    bug:
+        regex: 'bug[/-]'
+        tag: bugfix
+        source-branches: [{wellKnownBranchKey}]";
+            SetupConfigFileContent(text);
+            var config = configProvider.Provide(repoPath);
+
+            config.Branches["bug"].SourceBranches.ShouldBe(new List<string> { wellKnownBranchKey });
+        }
+
         [Test]
         public void CanProvideConfigForNewBranch()
         {