bank-account: use async Task for concurrency test

Author: ErikSchierboom

exercises/practice/bank-account/.meta/Generator.tpl

@@ -28,7 +28,7 @@ public class {{ testClass }}
 {
     {{- for test in tests }}
     [Fact{{ if !for.first }}(Skip = "Remove this Skip property to run this test"){{ end }}]
-    public void {{ test.testMethod }}()
+    public {{ if test.scenarios | array.contains "concurrent" }}async Task{{ else }}void{{ end }} {{ test.testMethod }}()
     {
         var account = new {{ testedClass }}();
         {{- for op in test.input.operations }}
@@ -51,7 +51,7 @@ public class {{ testClass }}
                         {{- end -}}
                     }
                 }));
-                Task.WaitAll(tasks.ToArray());
+                await Task.WaitAll(tasks.ToArray());
             }
             {{- else }}
             account.{{ op | to_call }};

exercises/practice/bank-account/BankAccountTests.cs

@@ -153,7 +153,7 @@ public void Cannot_deposit_negative()
     }
 
     [Fact(Skip = "Remove this Skip property to run this test")]
-    public void Can_handle_concurrent_transactions()
+    public async Task Can_handle_concurrent_transactions()
     {
         var account = new BankAccount();
         account.Open();
@@ -168,7 +168,7 @@ public void Can_handle_concurrent_transactions()
                     account.Withdraw(1m);
                 }
             }));
-            Task.WaitAll(tasks.ToArray());
+            await Task.WaitAll(tasks.ToArray());
         }
         Assert.Equal(0m, account.Balance);
     }