Fix error testing remote with echo

[TakodaS]

RemoteStart producing the following error:

Testing remote connection' failed.
stack traceback:
	.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:974: in function '_handle_job_completion'
	.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:1019: in function 'run_command'
	.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:142: in function '_setup_workspace_variables'
	.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:872: in function '_launch_neovim'
	.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:883: in function <.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:882>
	[C]: in function 'xpcall'
	.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:759: in function <.../remote-nvim.nvim/lua/remote-nvim/providers/provider.lua:758> 

FAILED JOB OUTPUT (SO FAR)
bash: -c: line 1: unexpected EOF while looking for matching `''
bash: -c: line 2: syntax error: unexpected end of file

I added an extra line of debug information to get the raw ssh command executed:

DEBUG Thu 11 Jul 2024 15:03:13 BST .repro/data/nvim/lazy/remote-nvim.nvim/lua/remote-nvim/providers/executor.lua:76: Executor running command  "ssh remote 'echo '\\''Hello'\\'''" with options {
  stdout_cb = <function 1>
}

The error is produced because "echo 'Hello'" is a nasty nested string that causes undefined behavior even with vim.fn.shellescape. Perhaps it's better to use "pwd"?

[amitds1997]

Thanks for raising this PR! Before we work on this further, I would like to discuss (in #157) and probably try to understand why an echo command did not work if it works on a lot of the system; as this might creep up later in future commands that we might add (and they for sure would be more complicated than echo "Hello World").

The reason for choosing echo is that it's universal (just like pwd).