Skip new tests that use Swift modules if the host’s SwiftPM doesn’t store .swiftmodule files in a subdirectory

This fixes a test failure when running tests using Xcode 15.3.

Author: ahoppen

Tests/SourceKitLSPTests/PublishDiagnosticsTests.swift

@@ -154,6 +154,7 @@ final class PublishDiagnosticsTests: XCTestCase {
 
   func testDiagnosticUpdatedAfterDependentModuleIsBuilt() async throws {
     try SkipUnless.longTestsEnabled()
+    try await SkipUnless.swiftpmStoresModulesInSubdirectory()
 
     let project = try await SwiftPMTestProject(
       files: [

Tests/SourceKitLSPTests/PullDiagnosticsTests.swift

@@ -182,6 +182,7 @@ final class PullDiagnosticsTests: XCTestCase {
 
   func testDiagnosticUpdatedAfterDependentModuleIsBuilt() async throws {
     try SkipUnless.longTestsEnabled()
+    try await SkipUnless.swiftpmStoresModulesInSubdirectory()
 
     let project = try await SwiftPMTestProject(
       files: [