[AIRFLOW-6689] Last changes for AIP-21

[mik-laj]

I propose following changes:

Move TestHiveCli to hooks.test_hive
Improve structure of Cassandra senssor testss
Move Cassandra hook tests to test_cassandra
Move operators.presto_to_mysql operators.mysql.operators.presto_to_mysql
Remove duplicate test_to_gcs_to_s3
Move MSSQL tests to providers.microsoft.mssql.operators
Move GenericTransfer tests to test_generic_transfer
Move PythonVirtualenvOperator to test_python
Move GCP system tests to providers.google.cloud
Move myqsl_to_hive tests from test_operators
Move myqsl tests from test_operators to test_mysql
Move myqsl tests from test_operators to test_mysql
Move postgres tests from test_operators to test_postgres

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Issue link: AIRFLOW-6689

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• Description above provides context of the change
• Commit message/PR title starts with [AIRFLOW-NNNN]. AIRFLOW-NNNN = JIRA ID^*
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[potiuk]

Please rebase to latest master @mik-laj - we had a failing master drama and there is much higher chance the build will succeed. We have still some intermittent errors, but there should be less often.

[codecov-io]

Codecov Report

Merging #7301 into master will increase coverage by 53.79%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #7301       +/-   ##
==========================================
+ Coverage    32.4%   86.2%   +53.79%     
==========================================
  Files         866     867        +1     
  Lines       40556   40559        +3     
==========================================
+ Hits        13144   34963    +21819     
+ Misses      27412    5596    -21816

Impacted Files	Coverage Δ
airflow/operators/presto_to_mysql.py	100% <100%> (+57.14%)	⬆️
...rflow/providers/mysql/operators/presto_to_mysql.py	100% <100%> (ø)
...irflow/providers/apache/cassandra/sensors/table.py	0% <0%> (-62.5%)	⬇️
...rflow/providers/apache/cassandra/sensors/record.py	0% <0%> (-58.83%)	⬇️
airflow/plugins_manager.py	86.98% <0%> (-1.45%)	⬇️
airflow/executors/executor_loader.py	78.94% <0%> (ø)	⬆️
airflow/bin/airflow	84.61% <0%> (ø)	⬆️
airflow/bin/cli.py	94.73% <0%> (+2.1%)	⬆️
airflow/kubernetes/pod_launcher.py	91.97% <0%> (+2.91%)	⬆️
... and 734 more

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