[xharness] Always build the test libraries before doing anything else. (#3601)

Fixes an issue where multiple projects would try to build the test libraries simultaneously.

tests/xharness/Jenkins.cs

@@ -874,6 +874,7 @@ namespace xharness
 					populating = false;
 				}).Wait ();
 				GenerateReport ();
+				BuildTestLibraries ();
 				if (!IsServerMode) {
 					foreach (var task in Tasks)
 						tasks.Add (task.RunAsync ());
@@ -892,6 +893,11 @@ namespace xharness
 			get { return Harness.JenkinsConfiguration == "server"; }
 		}
 
+		void BuildTestLibraries ()
+		{
+			ProcessHelper.ExecuteCommandAsync ("make", $"all -j{Environment.ProcessorCount} -C {StringUtils.Quote (Path.Combine (Harness.RootDirectory, "test-libraries"))}", MainLog, TimeSpan.FromMinutes (1)).Wait ();
+		}
+
 		Task RunTestServer ()
 		{
 			var server = new HttpListener ();