implement isPR and isCI logic in dotnet test suite

tried reusing logic directly from the TestRuntime class
but it is not as simple as just linking the file (as done
here, for ex: xamarin#6802)

there are objc dependencies that require additional references, so
to keep the changes minimal here i just reimplemented the 2 methods
that were necessary..

tests/common/TestRuntime.cs

@@ -143,8 +143,8 @@ public static bool IsPullRequest {
 				var pr = string.Equals(Environment.GetEnvironmentVariable ("BUILD_REASON"), "PullRequest", StringComparison.Ordinal);
 				is_pull_request = pr;
 			}
+			return is_pull_request.Value;
 		}
-		return is_pull_request.Value;
 	}
 
 	public static void IgnoreInCI (string message)

tests/dotnet/UnitTests/ProjectTest.cs

@@ -1,11 +1,8 @@
-using System.Runtime.InteropServices;
 using System.Diagnostics;
 using System.Xml;
 
 using Mono.Cecil;
 
-using Xamarin.Tests;
-
 #nullable enable
 
 namespace Xamarin.Tests {
@@ -1677,7 +1674,7 @@ public void SourcelinkTest (ApplePlatform platform, string runtimeIdentifiers)
 			// it is valid which will not pass until the commit has
 			// been merged in and actually exists on github.
 
-			if (!TestRuntime.IsCI || TestRuntime.IsPullRequest)
+			if (!IsInCI || IsPullRequest)
 				Assert.Ignore ("This test is disabled for local runs and Pull Requests.");
 
 			var project = "MySimpleApp";

tests/dotnet/UnitTests/TestBaseClass.cs

@@ -465,5 +465,28 @@ public void AssertThatLinkerDidNotExecute (ExecutionResult result)
 			Assert.That (output, Does.Not.Contain ("LinkerConfiguration:"), "Custom steps did not run as expected.");
 		}
 
+		static bool? is_in_ci;
+		public static bool IsInCI {
+			get {
+				if (!is_in_ci.HasValue) {
+					var in_ci = !string.IsNullOrEmpty (Environment.GetEnvironmentVariable ("BUILD_REVISION"));
+					in_ci |= !string.IsNullOrEmpty (Environment.GetEnvironmentVariable ("BUILD_SOURCEVERSION")); // set by Azure DevOps
+					is_in_ci = in_ci;
+				}
+				return is_in_ci.Value;
+			}
+		}
+
+		static bool? is_pull_request;
+		public static bool IsPullRequest {
+			get {
+				if (!is_pull_request.HasValue) {
+					var pr = string.Equals(Environment.GetEnvironmentVariable ("BUILD_REASON"), "PullRequest", StringComparison.Ordinal);
+					is_pull_request = pr;
+				}
+				return is_pull_request.Value;
+			}
+		}
+
 	}
 }