Active Record tests still depend on `capture`. Let's keep it for now.

This is a follow-up to 3121412

/cc @rafaelfranca

This will remove deprecation warnings from the PostgreSQL suite:

```
DEPRECATION WARNING: #capture(stream) is deprecated and will be removed in the next release. (called from capture at /Users/senny/Projects/rails/activesupport/lib/active_support/core_ext/kernel/reporting.rb:89)
/Users/senny/Projects/rails/activesupport/lib/active_support/core_ext/kernel/reporting.rb:89:in `capture'
  /Users/senny/Projects/rails/activerecord/test/cases/adapters/postgresql/composite_test.rb:73:in `ensure_warning_is_issued'
  /Users/senny/Projects/rails/activerecord/test/cases/adapters/postgresql/composite_test.rb:48:in `test_column'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:106:in `block (3 levels) in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:204:in `capture_exceptions'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:103:in `block (2 levels) in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:256:in `time_it'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:102:in `block in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:317:in `on_signal'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:276:in `with_info_handler'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest/test.rb:101:in `run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:759:in `run_one_method'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:293:in `run_one_method'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:287:in `block (2 levels) in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:286:in `each'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:286:in `block in run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:317:in `on_signal'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:306:in `with_info_handler'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:285:in `run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:149:in `block in __run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:149:in `map'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:149:in `__run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:126:in `run'
  /Users/senny/Projects/rails/.bundle/gems/minitest-5.3.3/lib/minitest.rb:55:in `block in autorun'
```

2 files changed, 23 insertions, 19 deletions

diff --git a/activerecord/test/cases/tasks/mysql_rake_test.rb b/activerecord/test/cases/tasks/mysql_rake_test.rb
index 572ad2ed18..3e3a2828f3 100644
--- a/activerecord/test/cases/tasks/mysql_rake_test.rb
+++ b/activerecord/test/cases/tasks/mysql_rake_test.rb
@@ -288,25 +288,6 @@ module ActiveRecord
         @configuration.merge('port' => 10000),
         filename)
     end
-
-    private
-
-    def capture(stream)
-      stream = stream.to_s
-      captured_stream = Tempfile.new(stream)
-      stream_io = eval("$#{stream}")
-      origin_stream = stream_io.dup
-      stream_io.reopen(captured_stream)
-
-      yield
-
-      stream_io.rewind
-      return captured_stream.read
-    ensure
-      captured_stream.close
-      captured_stream.unlink
-      stream_io.reopen(origin_stream)
-    end
   end
 
   class MySQLStructureLoadTest < ActiveRecord::TestCase
diff --git a/activerecord/test/cases/test_case.rb b/activerecord/test/cases/test_case.rb
index b6c5511849..80aaead6d9 100644
--- a/activerecord/test/cases/test_case.rb
+++ b/activerecord/test/cases/test_case.rb
@@ -1,10 +1,33 @@
 require 'active_support/test_case'
 
 module ActiveRecord
+  # The +capture+ helper was deprecated because it is not thread safe.
+  # Our own tests still depend on it.
+  module UndeprecateCapture
+    def capture(stream)
+      stream = stream.to_s
+      captured_stream = Tempfile.new(stream)
+      stream_io = eval("$#{stream}")
+      origin_stream = stream_io.dup
+      stream_io.reopen(captured_stream)
+
+      yield
+
+      stream_io.rewind
+      return captured_stream.read
+    ensure
+      captured_stream.close
+      captured_stream.unlink
+      stream_io.reopen(origin_stream)
+    end
+  end
+
   # = Active Record Test Case
   #
   # Defines some test assertions to test against SQL queries.
   class TestCase < ActiveSupport::TestCase #:nodoc:
+    include UndeprecateCapture
+
     def teardown
       SQLCounter.clear_log
     end