Fix test to allow IF NOT EXISTS in structure:dump

Before:

```
$ ruby -v
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux]

$ ruby -w  -Itest -Ilib -I../activesupport/lib -I../actionpack/lib -I../actionview/lib -I../activemodel/lib test/application/rake/multi_dbs_test.rb
Run options: --seed 28744

F

Failure:
ApplicationTests::RakeTests::RakeMultiDbsTest#test_db:migrate_and_db:structure:dump_and_db:structure:load_works_on_all_databases [test/application/rake/multi_dbs_test.rb:70]:
Expected /CREATE TABLE \"books\"/ to match "CREATE TABLE IF NOT EXISTS \"schema_migrations\" (\"version\" varchar NOT NULL PRIMARY KEY);\nCREATE TABLE IF NOT EXISTS \"ar_internal_metadata\" (\"key\" varchar NOT NULL PRIMARY KEY, \"value\" varchar, \"created_at\" datetime NOT NULL, \"updated_at\" datetime NOT NULL);\nCREATE TABLE IF NOT EXISTS \"books\" (\"id\" integer PRIMARY KEY AUTOINCREMENT NOT NULL, \"title\" varchar, \"created_at\" datetime NOT NULL, \"updated_at\" datetime NOT NULL);\nCREATE TABLE sqlite_sequence(name,seq);\nINSERT INTO \"schema_migrations\" (version) VALUES\n('20180416201805');\n\n\n".
```

railties/test/application/rake/multi_dbs_test.rb

@@ -67,8 +67,8 @@ def db_migrate_and_schema_dump_and_load(namespace, expected_database, format)
           else
             schema_dump = File.read("db/#{format}.sql")
             schema_dump_animals = File.read("db/animals_#{format}.sql")
-            assert_match(/CREATE TABLE \"books\"/, schema_dump)
-            assert_match(/CREATE TABLE \"dogs\"/, schema_dump_animals)
+            assert_match(/CREATE TABLE (?:IF NOT EXISTS )?\"books\"/, schema_dump)
+            assert_match(/CREATE TABLE (?:IF NOT EXISTS )?\"dogs\"/, schema_dump_animals)
           end
 
           rails "db:#{format}:load"