[test] properly isolate mysql_connection unit test

Author: kares

test/db/mysql/unit_test.rb

@@ -148,21 +148,16 @@ def initialize; end
       assert_equal false, config[:properties]['useSSL']
     end
 
-    def load_jdbc_mysql
-      require 'jdbc/mysql'
-    rescue LoadError
-      return nil
-    end
-
   end
 
   context 'connection (Jdbc::MySQL missing)' do
 
     module ::Jdbc; end
 
-    @@jdbc_mysql = ::Jdbc::MySQL rescue nil
-
+    @@jdbc_mysql = false
     def setup
+      load_jdbc_mysql
+      @@jdbc_mysql = ::Jdbc::MySQL rescue nil
       ::Jdbc.send :remove_const, :MySQL if @@jdbc_mysql
     end
 
@@ -224,4 +219,10 @@ def teardown
 
   end
 
+  def load_jdbc_mysql
+    require 'jdbc/mysql'
+  rescue LoadError
+    return nil
+  end
+
 end if defined? JRUBY_VERSION