Problem:
LSP basic_finish test modified in #27899 is flaky again after #28030.
Solution:
Run on_setup() immediately after setup using a before_init callback.
Sets `NVIM_APPNAME` for the lsp server instances and also for the
`exec_lua` environment to ensure local user config doesn't interfere
with the test cases.
My local `ftplugin/xml.lua` broke the LSP test cases about setting
`omnifunc` defaults.
The spec indicates that the response may be `null`, but it doesn't
really say what a `null` response means. Since neovim raises an error if
the response is `null`, I figured that ignoring it would be the safest
bet.
Co-authored-by: Mathias Fussenegger <f.mathias@zignar.net>
* credit to @smolck and @theHamsta for their contributions in laying the
groundwork for this feature and for their work on some of the helper
utility functions and tests
