neotest only discovers positions for new buffers after writing a file

[mrcjkb]

To reproduce:

:e foo.rs
:Neotest run "runs tests for the current file
:e bar.rs
:Neotest run "No positions found
:w
:Neotest run "runs tests for the current file

This is likely because rustaceanvim only sends LSP client requests for buffers to which there's a client attached.

Possible solutions:

• Query all files, using the first client with a maching workspace root [a first attempt at this was not successful]
• Add an autocmd to rediscover positions on BufEnter

[mrcjkb]

nvim-neotest/neotest#360