Add specs to `Ecto.Adapters.SQL.Sandbox` #1557

Job	Run time
mssql integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 2017)	1m 46s
mysql integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 5.7)	1m 44s
mssql integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 2019)	1m 45s
postgres integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 15.0)	1m 24s
mysql integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 8.0)	1m 47s
postgres integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 11.11)	1m 17s
unittest (1.15.6, 24.3.4.13)	14s
unittest (1.15.6, 26.1.2, lint)	14s
postgres integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 9.6)	1m 13s
postgres integration test (1.11.4-erlang-21.3.8.24-alpine-3.13.3, 9.5)	1m 26s
	12m 50s

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