Add init_memory_sqlite decorator

[waketzheng]

Description

Almost every file in examples include the following two lines:

        await Tortoise.init(db_url="sqlite://:memory:", modules={"models": ["__main__"]})
        await Tortoise.generate_schemas()

Motivation and Context

• [x] Proposal 1: Add a decorator to initial memory sqlite
• [ ] Proposal 2: Add a optional argument into run_async to init Tortoise
• Example1:

  
  from tortoise import fields, models, run_async
  from tortoise.contrib.test import init_memory_sqlite

class User(models.Model): id = fields.IntField(primary_key=True) name = fields.TextField()

@init_memory_sqlite async def run(): user = await User.create(name='J') print(user.name)

if name == 'main': run_async(run())

- Example2:
```py
# app/models.py
from tortoise import fields, models

class User(models.Model):
    id = fields.IntField(primary_key=True)
    name = fields.TextField()

# main.py
from tortoise import run_async
from tortoise.contrib.test import init_memory_sqlite

from app.models import User

@init_memory_sqlite(['app.models'])
async def run():
    user = await User.create(name='J')
    print(user.name)

if __name__ == '__main__':
    run_async(run())

How Has This Been Tested?

make ci

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added the changelog accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 9544850985

Details

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.07%

---

Totals
Change from base Build 9543663341:	0.02%
Covered Lines:	5881
Relevant Lines:	6505

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9560727240

Details

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.07%

---

Totals
Change from base Build 9543663341:	0.02%
Covered Lines:	5881
Relevant Lines:	6505

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9560904994

Details

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.07%

---

Totals
Change from base Build 9543663341:	0.02%
Covered Lines:	5881
Relevant Lines:	6505

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9561639055

Details

• 14 of 14 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.07%

---

Totals
Change from base Build 9543663341:	0.02%
Covered Lines:	5881
Relevant Lines:	6505

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9562578561

Details

• 16 of 16 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 89.071%

---

Totals
Change from base Build 9543663341:	0.02%
Covered Lines:	5882
Relevant Lines:	6506

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9645745307

Details

• 32 of 33 (96.97%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.008%) to 89.094%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/contrib/test/init.py	32	33	96.97%
<!--	Total:	32	33	96.97%	-->

Totals
Change from base Build 9642974390:	0.008%
Covered Lines:	5906
Relevant Lines:	6531

---

💛 - Coveralls