regression(test): diagnostics handler handler changedtick tracking should call handler only once if buffer changed in between callbacks

[mochaaP]

Fail	||	diagnostics handler handler changedtick tracking should call handler only once if buffer changed in between callbacks	
            test/spec/diagnostics_spec.lua:126: Expected to be called 1 time(s), but was called 2 time(s)
            
            stack traceback:
            	test/spec/diagnostics_spec.lua:126: in function <test/spec/diagnostics_spec.lua:116>

Minimal repro:

make test-file FILE=test/spec/diagnostics_spec.lua

Seems a plenary / nvim update broke the async generator

[mochaaP]

addressed by commit 16bdde6 for now

[mochaaP]

seealso: neovim-stuff/null-ls.nvim#3 (comment)