Add tsqlt.TestCase extended property to mark sprocs as tests

[jeremysimmons]

The tSQLt.Tests view defines a valid test case as one which starts with the name 'test'.
Add the ability for the sproc to have an associated extended property 'tsqlt.Test'.
Add a sproc to set this extended property 'tsqlt.SetTest @sprocname sysname'

Happy to do the work and send a pull request

[lizbaron]

@jeremysimmons , can you please say more about the use case?