Refect lua/scratch/gotest_switcher.lua from 597fa4fb36604a2ead62150df4c5ca184c3c3a1c

after/ftplugin/go.lua

···
 map("n", "<localleader>t", "<cmd>GoTestAdd<cr>", true)
 map("n", "<localleader>a", "<cmd>GoTestsAll<cr>", true)
 map("n", "<localleader>e", "<cmd>GoTestsExp<cr>", true)
+
+map("n", "<localleader>s", require("scratch.gotest").switch, true)

lua/scratch/gotest.lua

···
+local gotests = {}
+
+function gotests.switch()
+  local bufnr = vim.api.nvim_get_current_buf()
+  local fname_parts = vim.split(vim.api.nvim_buf_get_name(bufnr), "/")
+  local test_fname = fname_parts[#fname_parts]
+  if test_fname:find "_test.go" then
+    test_fname = test_fname:gsub("_test.go", ".go")
+  else
+    test_fname = test_fname:gsub(".go", "_test.go")
+  end
+
+  table.remove(fname_parts, #fname_parts)
+  local path = table.concat(fname_parts, "/") .. "/" .. test_fname
+
+  vim.cmd("edit " .. path)
+end
+
+return gotests