spec: don't clear database automatically

[maiha]

In current specs, every time we run one test case, we initialize 33 tables with 3 adapters in total.

# pseudo code
Spec.before_each do
  adapters.each do |adapter| # 3 times
    adapter::table1.clear    # 11 tables in total
    ...

This is expensive to run and is unnecessary work for most test cases. Therefore, execution of all tests is very slow and this cost will continue to increase each time additional functionality is added.

In this PR, I propose a policy to delete tables manually rather than automatically. When writing a test, we do not assume that the table is empty, we need to manually run the table as needed.

Benchmark

In my environment.

master

• 8m13s (with 6118 DB operations)

PR

• 43s (with 1012 DB operations)

FYI: This is a commits dependency for natural-key in my fork.

1. add spec for callbacks #154 add spec for callbacks (merged) 🎉
2. spec: don't clear database automatically #165 spec: don't clear database automatically (This PR)
3. add record statuses about persistence: new_record?, destroyed?, persisted? #153 add record statuses about persistence
4. Set the primary key to a new record #143 primary auto: false works as a natural key

Best regards,