Fix flaky test in plugins/linear-genome-view

[cmdcolin]

This attempts to fix a flaky test where the snapshot can be snapped in a Loading state

seen in this test on main branch
https://github.com/GMOD/jbrowse-components/actions/runs/3166318840/jobs/5155949977#step:5:1598

Also adds typescripting to say that addTrackConf, addSessionConf can accept a config model (instantiated) or config snapshot (JSON) and typescripts that particular flaky test

[codecov]

Codecov Report

Merging #3242 (8b77373) into main (a602dc9) will decrease coverage by 0.18%.
The diff coverage is 39.13%.

@@            Coverage Diff             @@
##             main    #3242      +/-   ##
==========================================
- Coverage   59.41%   59.22%   -0.19%     
==========================================
  Files         674      674              
  Lines       28783    28782       -1     
  Branches     7009     7008       -1     
==========================================
- Hits        17101    17046      -55     
- Misses      11407    11451      +44     
- Partials      275      285      +10     

Impacted Files	Coverage Δ
products/jbrowse-web/src/SessionLoader.ts	61.30% <7.69%> (+0.52%)	⬆️
products/jbrowse-web/src/sessionModelFactory.ts	59.42% <66.66%> (+1.14%)	⬆️
products/jbrowse-web/src/rootModel.ts	54.73% <100.00%> (+0.15%)	⬆️
plugins/data-management/src/ucsc-trackhub/model.js	30.76% <0.00%> (-46.16%)	⬇️
.../data-management/src/ucsc-trackhub/ucscTrackHub.js	4.68% <0.00%> (-28.13%)	⬇️
...eElementTypes/models/BaseConnectionModelFactory.ts	56.25% <0.00%> (-18.75%)	⬇️
products/jbrowse-web/src/util.ts	48.33% <0.00%> (-1.67%)	⬇️
packages/core/util/index.ts	83.41% <0.00%> (-0.47%)	⬇️

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