Run wasm tests without requiring threads

- this fixes flakiness in couple unit tests due to a spurious error that occurred when creating extra threads https://github.com/emscripten-core/emscripten/issues/12223 by completely disabling threads for unit tests.
- sync XHR in `vtkPreloadDataFileIntoMemory` is apparently possible when the mimetype is `text/plain` - https://stackoverflow.com/a/63920556/7102809
- Now, `vtkEmscripten.cmake` does not enable threads by default when `VTK_BUILD_TESTING` is `WANT` or `ON`.
- Make `VTK_USE_PTHREADS` honor the value of `VTK_WEBASSEMBLY_THREADS`. This correctly configures code in VTK which assumes pthreads exist.
- Remove pthread proxying and offscreen canvas link flags from unit tests as they're no longer required.