Add Swift Testing helper functions

In the process of converted XCTests to Swift Testing, add a few Swift
Testing helper functions equivalent to the XCTest equivalents.

Unfortunately, I'm unable to write automated tests that verify the new
helpers behave as expected as the Swift Testing framework does not
currently allow writing tests that verify an issue was recorded. This is
tracked under swiftlang/swift-testing#155

Author: bkhouri

Sources/TSCTestSupport/SwiftTestingHelpers.swift

@@ -0,0 +1,48 @@
+/*
+ This source file is part of the Swift.org open source project
+
+ Copyright (c) 2024 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+
+ See http://swift.org/LICENSE.txt for license information
+ See http://swift.org/CONTRIBUTORS.txt for Swift project authors
+*/
+
+import Testing
+
+import TSCBasic
+
+public func assertFileExists(
+    _ path: AbsolutePath,
+    sourceLocation: SourceLocation = #_sourceLocation
+) {
+    print("localFileSystem.isFile(path) ==> \(localFileSystem.isFile(path))")
+    #expect(
+        localFileSystem.isFile(path),
+        "Expected file doesn't exist: \(path)",
+        sourceLocation: sourceLocation
+    )
+}
+public func assertDirectoryExists(
+    _ path: AbsolutePath,
+    sourceLocation: SourceLocation = #_sourceLocation
+) {
+    #expect(
+        localFileSystem.isDirectory(path),
+        "Expected directory doesn't exist: \(path)",
+        sourceLocation: sourceLocation
+    )
+}
+
+public func assertNoDiagnostics(
+    _ engine: DiagnosticsEngine,
+    sourceLocation: SourceLocation = #_sourceLocation
+) {
+    let diagnostics = engine.diagnostics
+    let diags = diagnostics.map({ "- " + $0.description }).joined(separator: "\n")
+    #expect(
+        diagnostics.isEmpty,
+        "Found unexpected diagnostics: \n\(diags)",
+        sourceLocation: sourceLocation
+    )
+}