Tests: remove @testable import SourceKitLSPAPI

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

Author: compnerd

Tests/SourceKitLSPAPITests/SourceKitLSPAPITests.swift

@@ -17,7 +17,7 @@ import Build
 import PackageGraph
 
 import PackageModel
-@testable import SourceKitLSPAPI
+import SourceKitLSPAPI
 import SPMBuildCore
 import _InternalTestSupport
 import XCTest