Performance cops will be extracted from RuboCop to RuboCop Performance
when next RuboCop 0.68 will be released.
https://github.com/rubocop-hq/rubocop/issues/5977

RuboCop 0.67 is its transition period.

Since rails/rails repository uses Performance cops, This PR added
rubocop-performance gem to Gemfile.

And this PR fixes some offenses using the following auto-correct.

```console
% bundle exec rubocop -a

Offenses:

activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb:212:26:
C: [Corrected] Layout/SpaceAroundOperators: Operator =
> should be surrounded by a single space.
              "primary"  => { adapter: "sqlite3", database: "db/primary.sqlite3" }
                         ^^
activerecord/test/cases/connection_adapters/connection_handlers_multi_db_test.rb:239:26:
C: [Corrected] Layout/SpaceAroundOperators: Operator => should be
surrounded by a single space.
              "primary"  => { adapter: "sqlite3", database: "db/primary.sqlite3" }
                         ^^
actionview/test/template/resolver_shared_tests.rb:1:1: C: [Corrected]
Style/FrozenStringLiteralComment: Missing magic comment #
frozen_string_literal: true.
module ResolverSharedTests
^
actionview/test/template/resolver_shared_tests.rb:10:33: C: [Corrected]
Layout/SpaceAroundEqualsInParameterDefault: Surrounding space missing in
default value assignment.
  def with_file(filename, source="File at #{filename}")
                                ^
actionview/test/template/resolver_shared_tests.rb:106:5: C: [Corrected]
Rails/RefuteMethods: Prefer assert_not_same over refute_same.
    refute_same a, b
    ^^^^^^^^^^^

2760 files inspected, 5 offenses detected, 5 offenses corrected
```