Fix warnings about quotes

```
    warning: single-quoted strings represent charlists. Use ~c"" if you indeed want a charlist or use "" instead.
    You may run "mix format --migrate" to fix this warning automatically.
    │
 43 │     addr = :config.get('couch_httpd', 'bind_address', '127.0.0.1')
    │                        ~
    │
    └─ test/elixir/lib/step/start.ex:43:24

    warning: found quoted keyword "_id" but the quotes are not required. Note that keywords are always atoms, even when quoted. Similar to atoms, keywords made exclusively of ASCII letters, numbers, and underscores and not beginning with a number do not require quotes
    │
 91 │     docs ++ [%{"_id": "#{doc2._id}", "_rev": att_rev2}]
    │                ~
    │
    └─ test/elixir/test/compact_test.exs:91:16
```

Author: jiahuili430

test/elixir/lib/step/start.ex

@@ -40,13 +40,13 @@ defmodule Couch.Test.Setup.Step.Start do
     test_config = setup |> Setup.get(:test_config) |> Step.Config.get()
     protocol = test_config[:backdoor][:protocol] || "http"
     test_ctx = :test_util.start_couch(extra_apps)
-    addr = :config.get('couch_httpd', 'bind_address', '127.0.0.1')
+    addr = :config.get("couch_httpd", "bind_address", "127.0.0.1")
     port = :mochiweb_socket_server.get(:couch_httpd, :port)
     backdoor_url = "#{protocol}://#{addr}:#{port}"
     clustered_url =
       if :chttpd in extra_apps do
         protocol = test_config[:clustered][:protocol] || "http"
-        addr = :config.get('chttpd', 'bind_address', '127.0.0.1')
+        addr = :config.get("chttpd", "bind_address", "127.0.0.1")
         port = :mochiweb_socket_server.get(:chttpd, :port)
         "#{protocol}://#{addr}:#{port}"
       else

test/elixir/lib/step/user.ex

@@ -33,7 +33,7 @@ defmodule Couch.Test.Setup.Step.User do
 
   def setup(_setup, %__MODULE__{roles: roles} = step) do
     users_db = IO.chardata_to_string(
-      :config.get('chttpd_auth', 'authentication_db', '_users'))
+      :config.get("chttpd_auth", "authentication_db", "_users"))
     if not Utils.db_exists?(users_db) do
       on_exit fn ->
         :fabric.delete_db(users_db, [@admin])
@@ -46,7 +46,7 @@ defmodule Couch.Test.Setup.Step.User do
       name = Utils.random_name("admin")
       pass = Utils.random_password()
       :config.set(
-        'admins', String.to_charlist(name), String.to_charlist(pass), false)
+        "admins", String.to_charlist(name), String.to_charlist(pass), false)
       %{step |
         name: name,
         password: pass,

test/elixir/test/attachments_test.exs

@@ -131,7 +131,7 @@ defmodule AttachmentsTest do
     assert resp.status_code in [201, 202]
     rev = resp.body["rev"]
 
-    resp = Couch.delete("/#{db_name}/bin_doc/foo.txt", body: 'some payload', query: %{w: 3, rev: rev}, ibrowse: [{:max_sessions, 1}, {:max_pipeline_size, 1}])
+    resp = Couch.delete("/#{db_name}/bin_doc/foo.txt", body: "some payload", query: %{w: 3, rev: rev}, ibrowse: [{:max_sessions, 1}, {:max_pipeline_size, 1}])
     assert resp.status_code == 200
 
     resp = Couch.get("/", timeout: 1000, ibrowse: [{:max_sessions, 1}, {:max_pipeline_size, 1}])

test/elixir/test/compact_test.exs

@@ -88,7 +88,7 @@ defmodule CompactTest do
     assert resp.status_code in [201, 202]
 
     att_rev2 = resp.body["rev"]
-    docs ++ [%{"_id": "#{doc2._id}", "_rev": att_rev2}]
+    docs ++ [%{_id: "#{doc2._id}", _rev: att_rev2}]
   end
 
   defp delete(db, docs) do