fix `bin/test -a sqlite3_mem`.

We should not run MysqlAdapter tests when running with `sqlite3_mem`.
This also moves the test-case outside the MysqlAdapter namespace. This
will prevent the following error when running everything:

```
  1) Error:
TestAdapterWithInvalidConnection#test_inspect_on_Model_class_does_not_raise:
TypeError: superclass mismatch for class MysqlAdapter
```

1 files changed, 13 insertions, 19 deletions

diff --git a/activerecord/test/cases/adapters/mysql/explain_test.rb b/activerecord/test/cases/adapters/mysql/explain_test.rb
index f75386abb9..c44c1e6648 100644
--- a/activerecord/test/cases/adapters/mysql/explain_test.rb
+++ b/activerecord/test/cases/adapters/mysql/explain_test.rb
@@ -2,26 +2,20 @@ require "cases/helper"
 require 'models/developer'
 require 'models/computer'
 
-module ActiveRecord
-  module ConnectionAdapters
-    class MysqlAdapter
-      class ExplainTest < ActiveRecord::TestCase
-        fixtures :developers
+class MysqlExplainTest < ActiveRecord::MysqlTestCase
+  fixtures :developers
 
-        def test_explain_for_one_query
-          explain = Developer.where(:id => 1).explain
-          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
-          assert_match %r(developers |.* const), explain
-        end
+  def test_explain_for_one_query
+    explain = Developer.where(id: 1).explain
+    assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
+    assert_match %r(developers |.* const), explain
+  end
 
-        def test_explain_with_eager_loading
-          explain = Developer.where(:id => 1).includes(:audit_logs).explain
-          assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
-          assert_match %r(developers |.* const), explain
-          assert_match %(EXPLAIN for: SELECT `audit_logs`.* FROM `audit_logs` WHERE `audit_logs`.`developer_id` = 1), explain
-          assert_match %r(audit_logs |.* ALL), explain
-        end
-      end
-    end
+  def test_explain_with_eager_loading
+    explain = Developer.where(id: 1).includes(:audit_logs).explain
+    assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers` WHERE `developers`.`id` = 1), explain
+    assert_match %r(developers |.* const), explain
+    assert_match %(EXPLAIN for: SELECT `audit_logs`.* FROM `audit_logs` WHERE `audit_logs`.`developer_id` = 1), explain
+    assert_match %r(audit_logs |.* ALL), explain
   end
 end