[Karinon]

(last one for today)

Fixes three memory leaks:

• String variables: reading a string variable with getVariableArray or getSlicedVariableArray never freed the strings netCDF-C allocated, so memory grew with every read. They are now freed, as get() already did.
• fromBlobLazy: if a file failed to open (e.g. it isn't a netCDF file), the worker kept running with no way to stop it. It is now terminated, as fromUrl already does.
• Viewer: opening a new file or URL never closed the previous dataset, so each one left a worker running until the tab was closed. The previous dataset is now closed once the new one has loaded, and a dataset whose loading fails is closed too.

Tests

• New tests check that repeated string reads don't grow the WASM heap.
• The worker tests now check that the worker is stopped whenever opening fails, and when the dataset is closed.