Use new SPI for SourceKitLSPAPITests (#7468)

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

---------

Co-authored-by: Max Desiatov <m_desiatov@apple.com>

Author: bnbarham

Tests/SPMBuildCoreTests/PluginInvocationTests.swift

@@ -11,7 +11,10 @@
 //===----------------------------------------------------------------------===//
 
 import Basics
+
+@_spi(DontAdoptOutsideOfSwiftPMExposedForBenchmarksAndTestsOnly)
 @testable import PackageGraph
+
 import PackageLoading
 import PackageModel
 @testable import SPMBuildCore

Tests/SourceKitLSPAPITests/SourceKitLSPAPITests.swift

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