test: refactor test-util-inspect.js

* Enclose tests that used to introduce module-level variables into
  their own scopes.
* Replace ES5 anonymous functions with arrow functions where it makes
  sense.
* And make one arrow function a regular function thus fixing a bug in a
  getter inside an object created in "Array with dynamic properties"
  test.  This getter has never been invoked though, so the test hasn't been
  failing.
* Convert snake_case identifiers to camelCase.
* Make some variable names more readable.
* Replace regular expressions in maxArrayLength tests with simple
  assert.strictEquals() and assert(...endsWith()) checks, as suggested
  in <nodejs#11576 (comment)>.