add tests for use_data_table() (r-lib#994)

* add tests for `use_data_table()`

* remove context and mock install data.table

tests/testthat/test-use-data-table.R

@@ -0,0 +1,19 @@
+test_that("use_data_table() requires a package", {
+  scoped_temporary_project()
+  expect_usethis_error(use_data_table(), "not an R package")
+})
+
+test_that("use_data_table() Imports data.table", {
+  with_mock(
+    `usethis:::uses_roxygen` = function(base_path) TRUE,
+    `usethis:::is_installed` = function(pkg) TRUE, {
+      scoped_temporary_package()
+      use_data_table()
+      expect_match(desc::desc_get("Imports", proj_get()), "data.table")
+      datatable_doc <- readLines(proj_path("R", "utils-data-table.R"))
+      expect_match(datatable_doc, "#' @import data.table", all = FALSE)
+    }
+  )
+})
+
+