Convert tests to async tasks

Fix deprecated warnings

OhmGraphite.Test/GraphiteTest.cs

@@ -2,6 +2,7 @@ using DotNet.Testcontainers.Builders;
 using System;
 using System.Net.Http;
 using System.Threading;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace OhmGraphite.Test
@@ -9,7 +10,7 @@ namespace OhmGraphite.Test
     public class GraphiteTest
     {
         [Fact, Trait("Category", "integration")]
-        public async void InsertGraphiteTest()
+        public async Task InsertGraphiteTest()
         {
             var testContainersBuilder = new ContainerBuilder()
                 .WithDockerEndpoint(DockerUtils.DockerEndpoint())
@@ -50,7 +51,7 @@ namespace OhmGraphite.Test
         }
 
         [Fact, Trait("Category", "integration")]
-        public async void InsertTagGraphiteTest()
+        public async Task InsertTagGraphiteTest()
         {
             var testContainersBuilder = new ContainerBuilder()
                 .WithDockerEndpoint(DockerUtils.DockerEndpoint())

OhmGraphite.Test/PrometheusTest.cs

@@ -1,5 +1,6 @@
 using System.Collections.Generic;
 using System.Net.Http;
+using System.Threading.Tasks;
 using Prometheus;
 using Xunit;
 
@@ -8,7 +9,7 @@ namespace OhmGraphite.Test
     public class PrometheusTest
     {
         [Fact]
-        public async void PrometheusTestServer()
+        public async Task PrometheusTestServer()
         {
             var collector = new TestSensorCreator();
             var registry = PrometheusCollection.SetupDefault(collector);
@@ -23,7 +24,7 @@ namespace OhmGraphite.Test
         }
 
         [Fact]
-        public async void PrometheusNicGuid()
+        public async Task PrometheusNicGuid()
         {
             var collector = new NicGuidSensor();
             var registry = PrometheusCollection.SetupDefault(collector);

OhmGraphite.Test/TimescaleTest.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Threading.Tasks;
 using DotNet.Testcontainers.Builders;
 using Npgsql;
 using Xunit;
@@ -8,7 +9,7 @@ namespace OhmGraphite.Test
     public class TimescaleTest
     {
         [Fact, Trait("Category", "integration")]
-        public async void CanSetupTimescale()
+        public async Task CanSetupTimescale()
         {
             var testContainersBuilder = new ContainerBuilder()
                 .WithDockerEndpoint(DockerUtils.DockerEndpoint())