Add unit tests to the GA version of the datamigration sdk

[vchske]

No description provided.

[azuresdkci]

Can one of the admins verify this patch?

[codecov-io]

Codecov Report

Merging #2561 into master will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2561      +/-   ##
==========================================
+ Coverage   55.17%   55.24%   +0.07%     
==========================================
  Files        6628     6833     +205     
  Lines      144734   147701    +2967     
==========================================
+ Hits        79854    81602    +1748     
- Misses      64880    66099    +1219

Impacted Files	Coverage Δ
...gmt/datamigration/models/database_file_info_py3.py	33.33% <0%> (ø)
...re/mgmt/datamigration/models/resource_sku_costs.py	55.55% <0%> (ø)
.../models/connect_to_target_sql_db_task_input_py3.py	71.42% <0%> (ø)
...s/migrate_sql_server_sql_mi_task_properties_py3.py	55.55% <0%> (ø)
...ct_to_source_sql_server_task_output_login_level.py	41.66% <0%> (ø)
...te_sql_server_sql_mi_task_output_database_level.py	33.33% <0%> (ø)
...o_source_sql_server_task_output_agent_job_level.py	38.46% <0%> (ø)
.../datamigration/models/available_service_sku_py3.py	50% <0%> (ø)
.../azure/mgmt/datamigration/models/database_table.py	62.5% <0%> (ø)
...odels/migrate_sql_server_sql_mi_task_properties.py	55.55% <0%> (ø)
... and 195 more

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[lmazuel]

LGTM, thank you!