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, the Swift Testing framework does not currently allow
writing tests that verify an issue was recorded.
swiftlang/swift-testing#155

Author: bkhouri

Sources/TSCTestSupport/SwiftTestingHelpers.swift

@@ -0,0 +1,17 @@
+import Testing
+
+import TSCBasic
+
+public func assertFileExists(_ path: AbsolutePath) {
+    print("localFileSystem.isFile(path) ==> \(localFileSystem.isFile(path))")
+    #expect(localFileSystem.isFile(path), "Expected file doesn't exist: \(path)")
+}
+public func assertDirectoryExists(_ path: AbsolutePath) {
+    #expect(localFileSystem.isDirectory(path), "Expected directory doesn't exist: \(path)")
+}
+
+public func assertNoDiagnostics(_ engine: DiagnosticsEngine) {
+    let diagnostics = engine.diagnostics
+    let diags = diagnostics.map({ "- " + $0.description }).joined(separator: "\n")
+    #expect(diagnostics.isEmpty, "Found unexpected diagnostics: \n\(diags)")
+}