We make the text plugin's is_active method short-circuit if it detects
relevant data present within the multiplexer. Subsequently, there would
be no more need to check for data stored within plugin assets.

Modified tests to check for correct behavior when the plugin detects
relevant data within the multiplexer.

Specifically, in that case,

1. The thread that seeks plugin assets data should not start.
2. The plugin should be active despite how that thread had not started.
3. The tags route should respond with multiplexer data.