Tests: add helper Stress attribute for stress testing

tests/FSharp.Compiler.ComponentTests/CompilerService/AsyncMemoize.fs

@@ -422,18 +422,19 @@ let ``Cancel running jobs with the same key`` () =
     // detach requests from their running computations
     cts.Cancel()
 
+    // Cancel the Get reequests, leaving the jobs running unawaited.
     for job in jobsToCancel do assertTaskCanceled job
 
+    // Start another request.
     let job = cache.Get(key 11, work) |> Async.StartAsTask
 
     // up til now the jobs should have been running unobserved
     let current = eventsWhen events (received Requested)
     Assert.Equal(0, current |> countOf Canceled)
 
-    // waitUntil events (countOf Canceled >> (=) 10)
-
-    waitUntil events (received Started)
+    waitUntil events (countOf Started >> (=) 11)
 
+    // Allow the single current request to finish.
     jobCanContinue.Set() |> ignore
 
     job.Wait()

tests/FSharp.Test.Utilities/XunitHelpers.fs

@@ -21,6 +21,14 @@ open OpenTelemetry.Trace
 [<AttributeUsage(AttributeTargets.Class ||| AttributeTargets.Method, AllowMultiple = false)>]
 type RunTestCasesInSequenceAttribute() = inherit Attribute()
 
+// Helper for stress testing.
+// Runs a test case many times in parallel.
+// Example usage: [<Theory; Repeat(Count = 1000)>]
+type RepeatAttribute([<ParamArray>] data: obj array) =
+    inherit DataAttribute()
+    member val Count = 1 with get, set
+    override this.GetData _ = Seq.init this.Count (fun i -> [| yield! data; yield box i |])
+
 #if XUNIT_EXTRAS
 
 // To use xUnit means to customize it. The following abomination adds 2 features: