Reland "[clangd][test] Disable a particular testcase in...

Reland "[clangd][test] Disable a particular testcase in FindExplicitReferencesTest when LLVM_ENABLE_EXPENSIVE_CHECKS""

The test got re-enabled after d54d71b6 landed.

However it seems that the order is still not deterministic as it
currently passes with -DLLVM_ENABLE_EXPENSIVE_CHECKS=OFF but randomly
fails with expensive checks ON.