test: cover the survivor lines of the dead-code removal (#98)

Address self-review nits on PR #99:

- nit 1: drop the inert `file.create(fake_tar)` + `on.exit`
  setup. With `update_tar_gz = FALSE`, the `if (update_tar_gz)`
  block is skipped and `list.files` never reads the fake file -
  the setup did nothing.
- nit 2: add a second test for the `build_from_source = FALSE,
  update_tar_gz = TRUE` path. Mocks `pkgbuild::build` (returns a
  fake tar.gz path) and `usethis::use_build_ignore` (no-op), then
  asserts both bindings were actually called - the survivor lines
  of the dead-code removal (`use_build_ignore(files = out)` and
  the `cat_green_tick(...)` success message) are now exercised.

This brings the previously-claimed-but-untested 0% region to
covered.

Author: VincentGuyader

tests/testthat/test-dock_from_desc.R

@@ -157,11 +157,8 @@ withr::with_dir(
       )
     })
 
-    test_that("dock_from_desc(build_from_source = FALSE) copies a prebuilt tar.gz", {
+    test_that("dock_from_desc(build_from_source = FALSE, update_tar_gz = FALSE) copies a prebuilt tar.gz", {
       skip_if(is_rdevel, "skip on R-devel")
-      fake_tar <- "fakepkg_0.0.0.tar.gz"
-      file.create(fake_tar)
-      on.exit(unlink(fake_tar), add = TRUE)
       my_dock <- testthat::with_mocked_bindings(
         code = dock_from_desc(
           file.path(".", "DESCRIPTION__"),
@@ -176,6 +173,36 @@ withr::with_dir(
       expect_match(df, "rm /app.tar.gz", fixed = TRUE)
       expect_false(grepl("mkdir /build_zone", df, fixed = TRUE))
     })
+
+    test_that("dock_from_desc(build_from_source = FALSE, update_tar_gz = TRUE) builds a fresh tar.gz", {
+      skip_if(is_rdevel, "skip on R-devel")
+      build_called <- FALSE
+      use_build_ignore_called <- FALSE
+      my_dock <- testthat::with_mocked_bindings(
+        code = dock_from_desc(
+          file.path(".", "DESCRIPTION__"),
+          build_from_source = FALSE,
+          update_tar_gz = TRUE
+        ),
+        get_sysreqs = function(...) character(0),
+        build = function(path, dest_path, vignettes) {
+          build_called <<- TRUE
+          fake <- file.path(dest_path, "fakepkg_0.0.0.tar.gz")
+          file.create(fake)
+          fake
+        },
+        use_build_ignore = function(files) {
+          use_build_ignore_called <<- TRUE
+          invisible(TRUE)
+        }
+      )
+      # Both surviving lines from the dead-code-removal must execute on
+      # this code path.
+      expect_true(build_called)
+      expect_true(use_build_ignore_called)
+      df <- paste(my_dock$Dockerfile, collapse = "\n")
+      expect_match(df, "remotes::install_local", fixed = TRUE)
+    })
   }
 )