fix(lsp): prevent lsp tests from picking up local user config (#22606)

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.

test/functional/plugin/lsp_spec.lua

@@ -57,6 +57,7 @@ describe('LSP', function()
         return lsp.start_client {
           cmd_env = {
             NVIM_LOG_FILE = fake_lsp_logfile;
+            NVIM_APPNAME = "nvim_lsp_test";
           };
           cmd = {
             vim.v.progpath, '-l', fake_lsp_code, test_name;