Re-introduced warnings to test suite

[MichaelChirico]

I missed the warnings when testing during the #1199 updates -- a few places where I used sprintf() poorly, e.g.

── Error (expect_lint.R:56:3): expect_type_linter catches expect_true(is.<base type>) symbol ───────────────────────────
Error in `sprintf(lint_message_fmt, matched_function)`: (converted from warning) one argument not used by format 'expect_type(x, t) is better than expect_true(is.<t>(x))'
Backtrace:
     ▆
  1. ├─rlang::eval_tidy(code, args)
  2. └─lintr::expect_lint(...)
  3.   └─lintr::lint(file, ...) at lintr/R/expect_lint.R:56:2
  4.     └─lintr::get_lints(...) at lintr/R/lint.R:91:6
  5.       ├─lintr::flatten_lints(linter_fun(expr)) at lintr/R/lint.R:297:4
  6.       │ ├─base::structure(flatten_list(x, class = "lint"), class = "lints") at lintr/R/utils.R:22:2
  7.       │ └─lintr::flatten_list(x, class = "lint")
  8.       │   └─lintr assign_item(x) at lintr/R/utils.R:42:2
  9.       │     └─base::inherits(x, class) at lintr/R/utils.R:35:4
 10.       └─lintr linter_fun(expr)
 11.         └─lintr::xml_nodes_to_lints(...) at lintr/R/expect_type_linter.R:43:4

[AshesITR]

Time to run at least a non-required check with options(warn = 2)?