Remove 'if exists' from drop table statement then use `table_exists?`

Since 'drop table if exists' statement does not always work with
some databases such as Oracle.

activerecord/test/cases/associations/required_test.rb

@@ -18,8 +18,8 @@ class Child < ActiveRecord::Base
   end
 
   teardown do
-    @connection.execute("DROP TABLE IF EXISTS parents")
-    @connection.execute("DROP TABLE IF EXISTS children")
+    @connection.execute("DROP TABLE parents") if @connection.table_exists? 'parents'
+    @connection.execute("DROP TABLE children") if @connection.table_exists? 'children'
   end
 
   test "belongs_to associations are not required by default" do

activerecord/test/cases/attribute_decorators_test.rb

@@ -28,7 +28,7 @@ def type_cast_from_user(value)
 
     teardown do
       return unless @connection
-      @connection.execute 'DROP TABLE IF EXISTS attribute_decorators_model'
+      @connection.execute 'DROP TABLE attribute_decorators_model' if @connection.table_exists? 'attribute_decorators_model'
       Model.attribute_type_decorations.clear
       Model.reset_column_information
     end

activerecord/test/cases/migration/foreign_key_test.rb

@@ -29,8 +29,8 @@ class Astronaut < ActiveRecord::Base
 
       teardown do
         if defined?(@connection)
-          @connection.execute "DROP TABLE IF EXISTS astronauts"
-          @connection.execute "DROP TABLE IF EXISTS rockets"
+          @connection.execute "DROP TABLE astronauts" if @connection.table_exists? 'astronauts' 
+          @connection.execute "DROP TABLE rockets" if @connection.table_exists? 'rockets'
         end
       end
 

activerecord/test/cases/schema_dumper_test.rb

@@ -441,7 +441,7 @@ class SchemaDumperDefaultsTest < ActiveRecord::TestCase
 
   teardown do
     return unless @connection
-    @connection.execute 'DROP TABLE IF EXISTS defaults'
+    @connection.execute 'DROP TABLE defaults' if @connection.table_exists? 'defaults'
   end
 
   def test_schema_dump_defaults_with_universally_supported_types