reduce concurrent manifest loading test contention (#3954)

motivation: with manfiest loading async, the concurrent loading tests introduce significant contention on cpu since the underlying queue in unbounded

changes: use semaphore to limit the concurrency to max active cpus

Author: tomerd

Tests/PackageLoadingTests/PD_4_2_LoadingTests.swift

@@ -827,9 +827,9 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
 
     // run this with TSAN/ASAN to detect concurrency issues
     func testConcurrencyWithWarmup() throws {
-        let total = 1000
         try testWithTemporaryDirectory { path in
-
+            let total = 1000
+            let semaphore = DispatchSemaphore(value: Concurrency.maxOperations)
             let manifestPath = path.appending(components: "pkg", "Package.swift")
             try localFileSystem.writeFileContents(manifestPath) { stream in
                 stream <<< """
@@ -875,6 +875,7 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
 
             let sync = DispatchGroup()
             for _ in 0 ..< total {
+                semaphore.wait()
                 sync.enter()
                 delegate.prepare(expectParsing: false)
                 manifestLoader.load(
@@ -890,7 +891,10 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
                     observabilityScope: observability.topScope,
                     on: .global()
                 ) { result in
-                    defer { sync.leave() }
+                    defer {
+                        semaphore.signal()
+                        sync.leave()
+                    }
 
                     switch result {
                     case .failure(let error):
@@ -915,16 +919,17 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
 
     // run this with TSAN/ASAN to detect concurrency issues
     func testConcurrencyNoWarmUp() throws {
-        let total = 1000
         try testWithTemporaryDirectory { path in
-
+            let total = 1000
+            let semaphore = DispatchSemaphore(value: Concurrency.maxOperations)
             let observability = ObservabilitySystem.makeForTesting()
             let delegate = ManifestTestDelegate()
             let manifestLoader = ManifestLoader(toolchain: ToolchainConfiguration.default, cacheDir: path, delegate: delegate)
             let identityResolver = DefaultIdentityResolver()
 
             let sync = DispatchGroup()
             for _ in 0 ..< total {
+                semaphore.wait()
                 let random = Int.random(in: 0 ... total / 4)
                 let manifestPath = path.appending(components: "pkg-\(random)", "Package.swift")
                 if !localFileSystem.exists(manifestPath) {
@@ -958,7 +963,10 @@ class PackageDescription4_2LoadingTests: PackageDescriptionLoadingTests {
                     observabilityScope: observability.topScope,
                     on: .global()
                 ) { result in
-                    defer { sync.leave() }
+                    defer {
+                        semaphore.signal()
+                        sync.leave()
+                    }
 
                     switch result {
                     case .failure(let error):