Disable prelude import in language-server tests. (#7489)

This was substantially slowing down the overall test suite.

Fixes #7453

Assisted-by: Gemini via Antigravity

toolchain/language_server/testdata/text_document/import_prelude.carbon

@@ -2,6 +2,8 @@
 // Exceptions. See /LICENSE for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
+// EXTRA-ARGS: --prelude-import
+//
 // AUTOUPDATE
 // TIP: To test this file alone, run:
 // TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/language_server/testdata/text_document/import_prelude.carbon