Tests: remove @testable import XCBuildSupport

Use the newly extended APIs for testing and use a non-testable import of
the package, avoiding a recompilation.

Author: compnerd

Tests/XCBuildSupportTests/PIFBuilderTests.swift

@@ -21,7 +21,7 @@ import PackageModel
 import SPMBuildCore
 import _InternalTestSupport
 import _InternalBuildTestSupport
-@testable import XCBuildSupport
+import XCBuildSupport
 import XCTest
 
 final class PIFBuilderTests: XCTestCase {