Overriding vim.lsp.handlers['textDocument/formatting'] doesn't work here
because fake_lsp_server_setup() uses a table with __index to specify
client handlers, which takes priority over vim.lsp.handlers[], and as a
result the overridden handler is never called, and the test ends before
the vim.wait() even finishes.
Instead, set a global variable from the handler that is actually reached
(by vim.rpcrequest() from client handler), and avoid stopping the event
loop too early.