Hopefully fix the flymake bootstrap problem

Hopefully fix the flymake bootstrap problem

Immediately after M-x eglot, eglot's use of flymake was having trouble
detecting the first diagnostics sent from the server, resulting in an
annoying "Wait" in the mode-line.

* eglot.el (eglot--current-flymake-report-fn): Move up here.
(eglot--managed-mode): Set eglot--current-flymake-report-fn to nil
on teardown.
(eglot--maybe-activate-editing-mode): Simplify.
(eglot-handle-notification textDocument/publishDiagnostics): Set unreported-diagnostics to a cons.
(eglot-handle-notification eglot-rls window/progress): Simplify.

* eglot-tests.el (rls-basic-diagnostics): Simplify test.