Ensure setters are handled in object_usage_linter() test suite

[MichaelChirico]

          I don't know what this TODO is talking about, exactly. I tried these snippets and they lint as expected:

lint(linters = object_usage_linter(), "
foo <- function() {
  not_exists(x) <- 2
  not_exists(x)
}")
# <text>:3:3: warning: [object_usage_linter] no visible global function definition for 'not_exists<-'
#   not_exists(x) <- 2
#   ^~~~~~~~~~
# <text>:4:3: warning: [object_usage_linter] no visible global function definition for 'not_exists'
#   not_exists(x)
#   ^~~~~~~~~~
lint(linters = object_usage_linter(), "
not_exists <- function(x) x
foo <- function() {
  not_exists(x) <- 2
  not_exists(x)
}")
# <text>:4:3: warning: [object_usage_linter] no visible global function definition for 'not_exists<-'
#   not_exists(x) <- 2
#   ^~~~~~~~~~

So deleting and waiting for something to be reported in tracker if there's actually something there.

Originally posted by @MichaelChirico in #2439 (comment)

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The above should be added as regression tests, if not there already.