Parallelize tests by default. (#174)

This PR enables parallelization by default when using `swift test` and enables support for the `--no-parallel` flag in `swift test` added in swiftlang/swift-package-manager#7231.

The default value of `Configuration.isParallelizationEnabled` remains `false`. Other adopters are free to enable or disable parallelization by default.

`XCTestScaffold` is not affected. It is a shim used with XCTest in Swift 5.10 only and expects serial execution of tests.

Author: grynspan

Sources/Testing/Running/EntryPoint.swift

@@ -129,9 +129,10 @@ func configurationForSwiftPMEntryPoint(withArguments args: [String]) throws -> C
   // Do not consider the executable path AKA argv[0].
   let args = args.dropFirst()
 
-  // Parallelization
-  if args.contains("--parallel") {
-    configuration.isParallelizationEnabled = true
+  // Parallelization (on by default)
+  configuration.isParallelizationEnabled = true
+  if args.contains("--no-parallel") {
+    configuration.isParallelizationEnabled = false
   }
 
 #if !SWT_NO_FILE_IO

Tests/TestingTests/SwiftPMTests.swift

@@ -22,13 +22,16 @@ struct SwiftPMTests {
     #expect(!CommandLine.arguments().isEmpty)
   }
 
-  @Test("--parallel argument")
+  @Test("--parallel/--no-parallel argument")
   func parallel() throws {
     var configuration = try configurationForSwiftPMEntryPoint(withArguments: ["PATH"])
-    #expect(!configuration.isParallelizationEnabled)
+    #expect(configuration.isParallelizationEnabled)
 
     configuration = try configurationForSwiftPMEntryPoint(withArguments: ["PATH", "--parallel"])
     #expect(configuration.isParallelizationEnabled)
+
+    configuration = try configurationForSwiftPMEntryPoint(withArguments: ["PATH", "--no-parallel"])
+    #expect(!configuration.isParallelizationEnabled)
   }
 
   @Test("No --filter or --skip argument")