reorganize directories at the root of the repo

[BugenZhao]

There are too many directories residing at the root of the repository. For the sake of cleanliness, we should consider reorganize them.

For example, there are 4 directories for testing.

• backwards-compat-tests
• e2e_test
• e2e_test_slow
• integration_tests

Besides, all of the following are deployment-related and can be treated as "misc":

• docker
• standalone

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