tests(autocomplete): Ensure autocomplete has no chance for accidental collisions

[gadenbuie]

Fixes #826

    ── Failure ('test-auto-complete.R:3:3'): R auto complete finds runif vars ──────
    auto_complete_r("method not found") not equal to list().
    Length mismatch: comparison on first 0 components

From the above failing test, I'm assuming that length mismatch is the signal. Instead of returning no completions, "method not found" could be returning one or more completions, which could happen if there's a global variable running around that starts with found.

As far as I can tell, we don't have such a symbol in our tests, so this might be triggering due to CRAN test harness?

learnr:::auto_complete_r("method not found")
#> list()

found_in_cran_test_runner <- "foo"
learnr:::auto_complete_r("method not found")
#> [[1]]
#> [[1]][[1]]
#> [1] "found_in_cran_test_runner"
#> 
#> [[1]][[2]]
#> [1] FALSE