Use new SPI for SourceKitLSPAPITests

SPI was added here in swiftlang#7465, but missed adding it to this module.

Author: bnbarham

Tests/SourceKitLSPAPITests/SourceKitLSPAPITests.swift

@@ -12,7 +12,10 @@
 
 import Basics
 import Build
+
+@_spi(DontAdoptOutsideOfSwiftPMExposedForBenchmarksAndTestsOnly)
 import PackageGraph
+
 import PackageModel
 import SourceKitLSPAPI
 import SPMTestSupport