tests, favor `drop_table` and `:if_exists` over raw SQL.

We've replaced most querues using DROP TABLE in our tests already.
This patch replaces the last couple calls.

activerecord/test/cases/migration/create_join_table_test.rb

@@ -140,7 +140,7 @@ def with_table_cleanup
           tables_after = connection.tables - tables_before
 
           tables_after.each do |table|
-            connection.execute "DROP TABLE #{table}"
+            connection.drop_table table
           end
         end
     end

activerecord/test/cases/migrator_test.rb

@@ -312,7 +312,7 @@ def test_migrator_rollback
   def test_migrator_db_has_no_schema_migrations_table
     _, migrator = migrator_class(3)
 
-    ActiveRecord::Base.connection.execute("DROP TABLE schema_migrations")
+    ActiveRecord::Base.connection.drop_table "schema_migrations", if_exists: true
     assert_not ActiveRecord::Base.connection.table_exists?('schema_migrations')
     migrator.migrate("valid", 1)
     assert ActiveRecord::Base.connection.table_exists?('schema_migrations')

activerecord/test/schema/mysql2_specific_schema.rb

@@ -35,9 +35,7 @@
 END
 SQL
 
-  ActiveRecord::Base.connection.execute <<-SQL
-DROP TABLE IF EXISTS collation_tests;
-SQL
+  ActiveRecord::Base.connection.drop_table "collation_tests", if_exists: true
 
   ActiveRecord::Base.connection.execute <<-SQL
 CREATE TABLE collation_tests (
@@ -46,9 +44,7 @@
 ) CHARACTER SET utf8 COLLATE utf8_general_ci
 SQL
 
-  ActiveRecord::Base.connection.execute <<-SQL
-DROP TABLE IF EXISTS enum_tests;
-SQL
+  ActiveRecord::Base.connection.drop_table "enum_tests", if_exists: true
 
   ActiveRecord::Base.connection.execute <<-SQL
 CREATE TABLE enum_tests (

activerecord/test/schema/mysql_specific_schema.rb

@@ -46,9 +46,7 @@
 END
 SQL
 
-  ActiveRecord::Base.connection.execute <<-SQL
-DROP TABLE IF EXISTS collation_tests;
-SQL
+  ActiveRecord::Base.connection.drop_table "collation_tests", if_exists: true
 
   ActiveRecord::Base.connection.execute <<-SQL
 CREATE TABLE collation_tests (
@@ -57,9 +55,7 @@
 ) CHARACTER SET utf8 COLLATE utf8_general_ci
 SQL
 
-  ActiveRecord::Base.connection.execute <<-SQL
-DROP TABLE IF EXISTS enum_tests;
-SQL
+  ActiveRecord::Base.connection.drop_table "enum_tests", if_exists: true
 
   ActiveRecord::Base.connection.execute <<-SQL
 CREATE TABLE enum_tests (