Tests: remove @testable import LLBuildManifest

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

Author: compnerd

Tests/LLBuildManifestTests/LLBuildManifestTests.swift

@@ -13,7 +13,7 @@
 import struct Basics.AbsolutePath
 import class Basics.InMemoryFileSystem
 import class Foundation.PropertyListDecoder
-@testable import LLBuildManifest
+import LLBuildManifest
 import _InternalTestSupport
 import XCTest