Revert "Remove --enable-swift-testing _et al._ from swift build. … (#7790)

…(#7784)"

Author: grynspan

Sources/Commands/SwiftBuildCommand.swift

@@ -104,6 +104,21 @@ struct BuildCommandOptions: ParsableArguments {
     /// If should link the Swift stdlib statically.
     @Flag(name: .customLong("static-swift-stdlib"), inversion: .prefixedNo, help: "Link Swift stdlib statically")
     public var shouldLinkStaticSwiftStdlib: Bool = false
+
+    /// Which testing libraries to use (and any related options.)
+    @OptionGroup()
+    var testLibraryOptions: TestLibraryOptions
+
+    func validate() throws {
+        // If --build-tests was not specified, it does not make sense to enable
+        // or disable either testing library.
+        if !buildTests {
+            if testLibraryOptions.explicitlyEnableXCTestSupport != nil
+                || testLibraryOptions.explicitlyEnableSwiftTestingLibrarySupport != nil {
+                throw StringError("pass --build-tests to build test targets")
+            }
+        }
+    }
 }
 
 /// swift-build command namespace