Restore providers' unit tests marked as integration tests

[shahar1]

As I prepared to implement mssql integration tests (#38577), I noticed that existing unit tests had been moved to tests/integration (#38577, #28170), even though they are just pure unit tests. There's no setup_method for connection with an external service; everything is nicely mocked, and they run OK on Breeze locally (maybe a bit slow, but that's for another time)...why leave them out? :)
This PR relocates the unit tests back to their original directory, and also fixes a path in the test_project_structure.py (the original paths for the unit tests above were left in this file as is).
Also, test_bigquery_to_mssql.py was duplicated in the original path and in the providers' integration tests - I assume that by mistake or as a blueprint for the integration tests, but for now it's better be deleted to avoid confusion.

cc: @potiuk, @Taragolis

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[Taragolis]

Some history: It moved into integration because it have unsupported marker @pytest.mark.backend("mssql"): #38530, there was no check for unsupported DB Backends at that time.
That not a case anymore, because right now we check for supported backends: mysql, postgres, sqlite

All tests which marked as @pytest.mark.backend also automatically (implicitly) marked as @pytest.mark.db_test so it never run at non-db environment. Some modules or individual test might be required to be marked with pytest.mark.db_test, you could check locally in breeze which moved tests required this marker https://github.com/apache/airflow/blob/main/contributing-docs/testing/unit_tests.rst#db-and-non-db-tests

❯ breeze shell --backend none

root@a2b2d941fc72:/opt/airflow# pytest tests --skip-db-tests tests/providers/google/cloud/transfers/test_mssql_to_gcs.py tests/providers/google/cloud/transfers/test_mssql_to_gcs.py tests/providers/apache/hive/transfers/test_mssql_to_hive.py tests/providers/google/cloud/transfers/test_bigquery_to_mssql.py

[shahar1]

Some history: It moved into integration because it have unsupported marker @pytest.mark.backend("mssql"): #38530, there was no check for unsupported DB Backends at that time. That not a case anymore, because right now we check for supported backends: mysql, postgres, sqlite

All tests which marked as @pytest.mark.backend also automatically (implicitly) marked as @pytest.mark.db_test so it never run at non-db environment. Some modules or individual test might be required to be marked with pytest.mark.db_test, you could check locally in breeze which moved tests required this marker https://github.com/apache/airflow/blob/main/contributing-docs/testing/unit_tests.rst#db-and-non-db-tests

❯ breeze shell --backend none

root@a2b2d941fc72:/opt/airflow# pytest tests --skip-db-tests tests/providers/google/cloud/transfers/test_mssql_to_gcs.py tests/providers/google/cloud/transfers/test_mssql_to_gcs.py tests/providers/apache/hive/transfers/test_mssql_to_hive.py tests/providers/google/cloud/transfers/test_bigquery_to_mssql.py

I ran the tests with these commands and all ran smoothly.
As I previously stated, as of the present time, these are simple unit tests - so there shouldn't be a reason for them not to work without a backend.
I'll start working on the mssql integration tests soon.

[shahar1]

Closing to avoid conflicts with #39831 (I implemented the unit tests relocation there)