[clangd] Fix directory separators in infinite-instantiation.test

Fixing https://lab.llvm.org/buildbot/#/builders/123/builds/26934 caused by #92888. Third time the charm!

Author: Endilll

clang-tools-extra/clangd/test/infinite-instantiation.test

@@ -1,5 +1,5 @@
 // RUN: rm -rf %t.dir && mkdir -p %t.dir
-// RUN: echo '[{"directory": "%/t.dir", "command": "clang -ftemplate-depth=100 -x c++ %s", "file": "%/s"}]' > %t.dir/compile_commands.json
+// RUN: echo '[{"directory": "%/t.dir", "command": "clang -ftemplate-depth=100 -x c++ %/s", "file": "%/s"}]' > %t.dir/compile_commands.json
 // RUN: not clangd --compile-commands-dir=%t.dir -check=%s 2>&1 | FileCheck -strict-whitespace %s
 
 // CHECK: [template_recursion_depth_exceeded]