test: add unit test for out of bounds issue (#364)

tests/custom_surrounds_spec.lua

@@ -0,0 +1,41 @@
+local set_lines = function(lines)
+  vim.api.nvim_buf_set_lines(0, 0, -1, false, lines)
+end
+
+local check_lines = function(lines)
+  assert.are.same(lines, vim.api.nvim_buf_get_lines(0, 0, -1, false))
+end
+
+describe("custom surrounds", function()
+  before_each(function()
+    local bufnr = vim.api.nvim_create_buf(true, true)
+    vim.api.nvim_win_set_buf(0, bufnr)
+  end)
+
+  it("deletes surrounding code block", function()
+    require("nvim-surround").buffer_setup({
+      surrounds = {
+        c = {
+          add = function()
+            return { { '```', '' }, { '', '```' } }
+          end,
+          find = '(```[a-zA-Z]*\n)().-(\n```)()',
+          delete = '(```[a-zA-Z]*\n)().-(\n```)()',
+        },
+      },
+    })
+
+    set_lines({
+      [[```lua]],
+      [[print('foo')]],
+      [[```]],
+    })
+
+    -- The out of bounds issue doesn't occur on the first line
+    vim.cmd("normal j")
+    local success, err = pcall(vim.cmd.normal, "dsc")
+
+    assert(success, err)
+    check_lines({ "print('foo')" })
+  end)
+end)