Improve testing coverage

[philippjfr]

Lumen has grown a lot over time and a lot of the development was driven by the need to develop a particular feature very quickly. This has meant that code coverage hasn't been great and we are now playing catch up.

Sources

• Test both schema and data cache locks guard against multiple thread access
• Tests for JoinedSource
• Tests for DerivedSource
• Test Source.from_spec when loading from source in state.sources
• Write test linking Source parameter to a variable

Filters

• Write tests for all Filter types
• Write tests for Filter.sync_with_url feature
• Write test linking Filter parameter to a variable

Transforms

• Write tests for all Transform types (including SQLTransform)
• Write test linking Transform parameter to a variable

View

• Write tests for all View types (each test should confirm get_panel works and .update works)
• Write test linking View parameter to a variable

Auth

• Write tests for lumen.dashboard.Auth.authorized logic (mock pn.state.user_info)
• Write tests for lumen.auth.YamlAuthMapperPlugin transform